drm/amdkfd: svm range restore work deadlock when process exit

kfd_process_notifier_release flush svm_range_restore_work
which calls svm_range_list_lock_and_flush_work to flush deferred_list
work, but if deferred_list work mmput release the last user, it will
call exit_mmap -> notifier_release, it is deadlock with below backtrace.

Move flush svm_range_restore_work to kfd_process_wq_release to avoid
deadlock. Then svm_range_restore_work take task->mm ref to avoid mm is
gone while validating and mapping ranges to GPU.

Workqueue: events svm_range_deferred_list_work [amdgpu]
Call Trace:
 wait_for_completion+0x94/0x100
 __flush_work+0x12a/0x1e0
 __cancel_work_timer+0x10e/0x190
 cancel_delayed_work_sync+0x13/0x20
 kfd_process_notifier_release+0x98/0x2a0 [amdgpu]
 __mmu_notifier_release+0x74/0x1f0
 exit_mmap+0x170/0x200
 mmput+0x5d/0x130
 svm_range_deferred_list_work+0x104/0x230 [amdgpu]
 process_one_work+0x220/0x3c0

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reported-by: Ruili Ji <ruili.ji@amd.com>
Tested-by: Ruili Ji <ruili.ji@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>