drm/amd/pm: Implement get GFXOFF status for vangogh

Implement function to get current GFXOFF status for vangogh.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>