drm/imagination: Fix double call to drm_sched_entity_fini()

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

drm/imagination: Fix double call to drm_sched_entity_fini()

Call sequence of double call:
pvr_context_destroy
  pvr_context_kill_queues
    pvr_queue_kill
      drm_sched_entity_destroy
        drm_sched_entity_fini // here
  pvr_context_put
    kref_put(..., pvr_context_release)
      pvr_context_destroy_queues
        pvr_queue_destroy
          drm_sched_entity_fini // here

Call to drm_sched_entity_destroy() from pvr_context_kill_queues() calls
drm_sched_entity_flush() + drm_sched_entity_fini().
drm_sched_entity_flush() ensures all pending jobs are completed and
drm_sched_entity_fini() ensures no further submission is allowed as
per expectation from pvr_context_kill_queues(). Double call to
drm_sched_entity_fini() is misuse of the API so keep call only in
pvr_context_create() failure path.

Stack trace for issue with addition of refcounting for DRM entity
stats in commit fd177135f0e6 ("drm/sched: Account entity GPU time"):

[ 789.490527] ------------[ cut here ]------------
[ 789.490559] refcount_t: underflow; use-after-free.
[ 789.490657] WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xf4/0x144, CPU#0: kworker/u16:1/440
[ 789.490695] Modules linked in: powervr drm_gpuvm drm_exec gpu_sched drm_shmem_helper xhci_plat_hcd xhci_hcd dwc3 usbcore usb_common snd_soc_simple_card snd_soc_simple_card_utils sa2ul sha512 sha256 dwc3_am62 sha1 authenc rti_wdt libsha512 at24 sch_fq_codel fuse dm_mod ipv6
[ 789.490798] CPU: 0 UID: 0 PID: 440 Comm: kworker/u16:1 Not tainted 7.0.0-rc7-02049-g5e2c0700091b #22 PREEMPT
[ 789.490809] Hardware name: Texas Instruments AM625 SK (DT)
[ 789.490815] Workqueue: powervr-sched pvr_queue_fence_release_work [powervr]
[ 789.490868] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 789.490876] pc : refcount_warn_saturate+0xf4/0x144
[ 789.490884] lr : refcount_warn_saturate+0xf4/0x144
[ 789.490892] sp : ffff8000822cbcc0
[ 789.490895] x29: ffff8000822cbcc0 x28: 0000000000000000 x27: 0000000000000000
[ 789.490909] x26: 0000000000000000 x25: ffff800081b1e338 x24: ffff000004541405
[ 789.490922] x23: ffff000004bea950 x22: ffff00000042e400 x21: ffff000007123e30
[ 789.490935] x20: ffff000007123000 x19: ffff000007a80d50 x18: fffffffffffe7768
[ 789.490948] x17: 74736574202c6e6f x16: 697461746e656d65 x15: ffff800081b269f0
[ 789.490962] x14: 0000000000000030 x13: ffff800081b26a70 x12: 0000000000000211
[ 789.490975] x11: 00000000000000c0 x10: 0000000000000b50 x9 : ffff8000822cbb30
[ 789.490988] x8 : ffff0000014e7bb0 x7 : ffff00007725e780 x6 : 0000000372a05f49
[ 789.491001] x5 : 0000000000000000 x4 : 0000000000000001 x3 : 0000000000000010
[ 789.491013] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff0000014e7000
[ 789.491027] Call trace:
[ 789.491032] refcount_warn_saturate+0xf4/0x144 (P)
[ 789.491043] drm_sched_entity_fini+0x164/0x18c [gpu_sched]
[ 789.491081] pvr_queue_destroy+0x64/0x134 [powervr]
[ 789.491110] pvr_context_destroy_queues+0x34/0x64 [powervr]
[ 789.491138] pvr_context_release+0x70/0xac [powervr]
[ 789.491166] pvr_context_put.part.0+0x5c/0x7c [powervr]
[ 789.491193] pvr_context_put+0x14/0x24 [powervr]
[ 789.491221] pvr_queue_fence_release_work+0x20/0x38 [powervr]
[ 789.491249] process_one_work+0x160/0x4c4
[ 789.491264] worker_thread+0x188/0x310
[ 789.491276] kthread+0x130/0x13c
[ 789.491287] ret_from_fork+0x10/0x20
[ 789.491300] ---[ end trace 0000000000000000 ]---