drm/amdgpu: disable vcn power control for now

The dpm control for vcn has been moved to firmware, kernel always spins
"amdgpu: [powerplay] pp_dpm_powergate_uvd was not implemented", each
time when application runs, disable it for now till pg/cg. Also remove
clock manual setting when dpm disabled, which was inherited from uvd.

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>