drm/amdgpu: correct clock gating settings on feature unsupported

Clock gating setting is still performed even when the corresponding
CG feature is not supported. And the tricky part is disablement is
actually performed no matter for enablement or disablement request.
That seems not logically right.
Considering HW should already properly take care of the CG state, we
will just skip the corresponding clock gating setting when the feature
is not supported.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>