drm/amdgpu: Add the missing error handling for xa_store() call

Add the missing error handling for xa_store() call in the function
amdgpu_userq_fence_driver_alloc().

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>