drm/amdgpu: rework S3/S4/S0ix state handling

Set flags at the top level pmops callbacks to track
state.  This cleans up the current set of flags and
properly handles S4 on S0ix capable systems.

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