drm/amdgpu: add FGCG support

Add the CG flag for Fine Grained Clock Gating.

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