drm/i915/gt: Fix timeline left held on VMA alloc error

In the Linux kernel, the following vulnerability has been resolved:

drm/i915/gt: Fix timeline left held on VMA alloc error

The following error has been reported sporadically by CI when a test
unbinds the i915 driver on a ring submission platform:

<4> [239.330153] ------------[ cut here ]------------
<4> [239.330166] i915 0000:00:02.0: [drm] drm_WARN_ON(dev_priv->mm.shrink_count)
<4> [239.330196] WARNING: CPU: 1 PID: 18570 at drivers/gpu/drm/i915/i915_gem.c:1309 i915_gem_cleanup_early+0x13e/0x150 [i915]
...
<4> [239.330640] RIP: 0010:i915_gem_cleanup_early+0x13e/0x150 [i915]
...
<4> [239.330942] Call Trace:
<4> [239.330944] <TASK>
<4> [239.330949] i915_driver_late_release+0x2b/0xa0 [i915]
<4> [239.331202] i915_driver_release+0x86/0xa0 [i915]
<4> [239.331482] devm_drm_dev_init_release+0x61/0x90
<4> [239.331494] devm_action_release+0x15/0x30
<4> [239.331504] release_nodes+0x3d/0x120
<4> [239.331517] devres_release_all+0x96/0xd0
<4> [239.331533] device_unbind_cleanup+0x12/0x80
<4> [239.331543] device_release_driver_internal+0x23a/0x280
<4> [239.331550] ? bus_find_device+0xa5/0xe0
<4> [239.331563] device_driver_detach+0x14/0x20
...
<4> [357.719679] ---[ end trace 0000000000000000 ]---

If the test also unloads the i915 module then that's followed with:

<3> [357.787478] =============================================================================
<3> [357.788006] BUG i915_vma (Tainted: G U W N ): Objects remaining on __kmem_cache_shutdown()
<3> [357.788031] -----------------------------------------------------------------------------
<3> [357.788204] Object 0xffff888109e7f480 @offset=29824
<3> [357.788670] Allocated in i915_vma_instance+0xee/0xc10 [i915] age=292729 cpu=4 pid=2244
<4> [357.788994] i915_vma_instance+0xee/0xc10 [i915]
<4> [357.789290] init_status_page+0x7b/0x420 [i915]
<4> [357.789532] intel_engines_init+0x1d8/0x980 [i915]
<4> [357.789772] intel_gt_init+0x175/0x450 [i915]
<4> [357.790014] i915_gem_init+0x113/0x340 [i915]
<4> [357.790281] i915_driver_probe+0x847/0xed0 [i915]
<4> [357.790504] i915_pci_probe+0xe6/0x220 [i915]
...

Closer analysis of CI results history has revealed a dependency of the
error on a few IGT tests, namely:
- igt@api_intel_allocator@fork-simple-stress-signal,
- igt@api_intel_allocator@two-level-inception-interruptible,
- igt@gem_linear_blits@interruptible,
- igt@prime_mmap_coherency@ioctl-errors,
which invisibly trigger the issue, then exhibited with first driver unbind
attempt.

All of the above tests perform actions which are actively interrupted with
signals. Further debugging has allowed to narrow that scope down to
DRM_IOCTL_I915_GEM_EXECBUFFER2, and ring_context_alloc(), specific to ring
submission, in particular.

If successful then that function, or its execlists or GuC submission
equivalent, is supposed to be called only once per GEM context engine,
followed by raise of a flag that prevents the function from being called
again. The function is expected to unwind its internal errors itself, so
it may be safely called once more after it returns an error.

In case of ring submission, the function first gets a reference to the
engine's legacy timeline and then allocates a VMA. If the VMA allocation
fails, e.g. when i915_vma_instance() called from inside is interrupted
with a signal, then ring_context_alloc() fails, leaving the timeline held
referenced. On next I915_GEM_EXECBUFFER2 IOCTL, another reference to the
timeline is got, and only that last one is put on successful completion.
As a consequence, the legacy timeline, with its underlying engine status
page's VMA object, is still held and not released on driver unbind.

Get the legacy timeline only after successful allocation of the context
engine's VMA.

v2: Add a note on other submission methods (Krzysztof Karas):
Both execlists and GuC submission use lrc_alloc() which seems free
from a similar issue.

(cherry picked from commit cc43422b3cc79eacff4c5a8ba0d224688ca9dd4f)