drm/amdgpu: refine uvd 5.0 clock gate feature.

1. fix uvd cg status not correct.
2. fix uvd pg can't work on tonga.
3. enable uvd mgcg.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>