drm/amd/display: implement T12 compliance

[why]
When OS reboots, and panel is turned off, T12 may not be maintained.
T12 is defined as the interval between VDDC off (occurs at shutdown) and
the next VDDC on (occurs when eDP is POST-ed)

[how]
DC already tracks panel power off time.  Add a DC interface which DM can
call during shutdown.  Ideally this should be as late as possible during
the shutdown sequence so the extra delay is minimal.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jun Lei <jun.lei@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>