drm/amd/pm: skip setting gfx cgpg in the s0ix suspend-resume

In the s0ix entry need retain gfx in the gfxoff state,so here need't
set gfx cgpg in the S0ix suspend-resume process. Moreover move the S0ix
check into SMU12 can simplify the code condition check.

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