drm/amdgpu: fix user fence handling

This fixes a random corruption under memory pressure. We need to fence
the BO for the user fence as well, otherwise it might be swapped out
and the GPU could write the fence value to an undesired location.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org