drm/amd/amdgpu: add uvd6.0 clock gating support. (v2)

v2: fix bug in register mask setting.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>