drm/amdgpu: add set_gfx_cgpg implement (v2)

add set_gfx_cgpg implement

v2: check if using sw_smu (Alex)

Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>