drm/amd/pp: Add gfx pg support in smu through set_powergating_by_smu

gfx ip block can call set_powergating_by_smu to set gfx pg state if
necessary.

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