drm/amd: add support to check whether the system is set to s3

This will be used to help make decisions on what to do in
misconfigured systems.

v2: squash in semicolon fix from Stephen Rothwell

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>