drm/amdgpu: Few optimization and fixes for userq fence driver

Few optimization and fixes for userq fence driver.

v1:(Christian):
  - Remove unnecessary comments.
  - In drm_exec_init call give num_bo_handles as last parameter it would
    making allocation of the array more efficient
  - Handle return value of __xa_store() and improve the error handling of
    amdgpu_userq_fence_driver_alloc().

v2:(Christian):
   - Revert userq_xa xarray init to XA_FLAGS_LOCK_IRQ.
   - move the xa_unlock before the error check of the call xa_err(__xa_store())
     and moved this change to a separate patch as this is adding a missing error
     handling.
   - Removed the unnecessary comments.

Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>