drm/amd/amdgpu: Enable clockgating in UVD6 for Stoney

This patch enables clockgating for the UVD6 block in Stoney.

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