drm/radeon: fix UVD fence emit

Currently doesn't matter cause we allocate the fence in the
lower 265MB anyway.

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