drm/amdgpu: fix NULL pointer dereference in amdgpu_gmc_filter_faults_remove

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

drm/amdgpu: fix NULL pointer dereference in amdgpu_gmc_filter_faults_remove

On APUs such as Raven and Renoir (GC 9.1.0, 9.2.2, 9.3.0), the ih1 and
ih2 interrupt ring buffers are not initialized. This is by design, as
these secondary IH rings are only available on discrete GPUs. See
vega10_ih_sw_init() which explicitly skips ih1/ih2 initialization when
AMD_IS_APU is set.

However, amdgpu_gmc_filter_faults_remove() unconditionally uses ih1 to
get the timestamp of the last interrupt entry. When retry faults are
enabled on APUs (noretry=0), this function is called from the SVM page
fault recovery path, resulting in a NULL pointer dereference when
amdgpu_ih_decode_iv_ts_helper() attempts to access ih->ring[].

The crash manifests as:

BUG: kernel NULL pointer dereference, address: 0000000000000004
RIP: 0010:amdgpu_ih_decode_iv_ts_helper+0x22/0x40 [amdgpu]
Call Trace:
amdgpu_gmc_filter_faults_remove+0x60/0x130 [amdgpu]
svm_range_restore_pages+0xae5/0x11c0 [amdgpu]
amdgpu_vm_handle_fault+0xc8/0x340 [amdgpu]
gmc_v9_0_process_interrupt+0x191/0x220 [amdgpu]
amdgpu_irq_dispatch+0xed/0x2c0 [amdgpu]
amdgpu_ih_process+0x84/0x100 [amdgpu]

This issue was exposed by commit 1446226d32a4 ("drm/amdgpu: Remove GC HW
IP 9.3.0 from noretry=1") which changed the default for Renoir APU from
noretry=1 to noretry=0, enabling retry fault handling and thus
exercising the buggy code path.

Fix this by adding a check for ih1.ring_size before attempting to use
it. Also restore the soft_ih support from commit dd299441654f ("drm/amdgpu:
Rework retry fault removal"). This is needed if the hardware doesn't
support secondary HW IH rings.

v2: additional updates (Alex)

(cherry picked from commit 6ce8d536c80aa1f059e82184f0d1994436b1d526)