drm/amdgpu: refine uvd5.0/6.0 code.

1. delete redundant cg pg mask check.
   pg mask use to ctrl power on/down uvd.
   not start/stop uvd.
   cg mask will be check when enable mgcg.
2. no need to start uvd when initializ.
   when ring test/ib test/encode, uvd was enabled.
   when uvd idle, uvd was stopped.
3. chang cg pg sequence in powerplay.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>