drm/amdgpu/userq: make sure queue is valid in the hang_detect_work

drm/amdgpu/userq: make sure queue is valid in the hang_detect_work

Thread 1: Running amdgpu_userq_destroy which eventually remove
the queue from door bell and set userq_mgr = NULL.

Thread2: An interrupt might have scheduled the hang_detect_work
which still need userq_mgr to be valid but could get an NULL
ptrs.

To fix that make sure we cancel the hang_detect_work again before
setting userq_mgr to NULL.

Along with that we also need all the queue va to remain valid till
we could be running anything on the queue and hence moving the
userq_va post hang_detect handler is cancelled.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>