drm/amdgpu/vce: simplify vce instance setup

Set the me instance in early init and use that rather than
calculating the instance based on the ring pointer.

Reviewed-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>