drm/amd/amdgpu: Add SW clock gating support to UVD 5 and 6

This patch adds support for software clock gating to UVD 5
and UVD 6 blocks with a preliminary commented out hardware
gating routine.

Currently hardware gating does not work so it's not activated.

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>