drm/amdgpu: avoid to perform undesired clockgating operation

Make sure the clockgating feature is supported before action.
Otherwise, the feature may be disabled unexpectedly on enablement
request.

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