drm/amdgpu: rework userq fence driver alloc/destroy

drm/amdgpu: rework userq fence driver alloc/destroy

The correct fix is to tie the global xa entry lifetime to the
queue lifetime: insert in amdgpu_userq_create() and erase in
amdgpu_userq_cleanup(), both at the well-defined doorbell_index key,
making the operation O(1) and resolve the fence driver UAF problem
by binding the userq driver fence to per queue.

v2: clean up the local variables initialization. (Christian)

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>