drm/amdgpu: expose only the first UVD instance for now

Going to completely rework the context to ring mapping with Nayan's GSoC
work, but for now just stopping to expose the second UVD instance should
do it.

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>