drm/amd/powerplay: add parts of system clock gating support for Fiji. (v2)

Removed fiji_mgcg_cgcg_init that is affected and redundant for new implementation.

v2: re-add mgcg_cgcg init

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Eric Huang <JinHuiEric.Huang@amd.com>