drm/amd/display: Add condition to sync eDP SW status and HW status

[Why]
Need to disable EDP backlight when enter S4 with EDP only
and resume from S4 with secondary only.

[How]
Align the real hw and sw state via vBios scratch register in
function enable_accelerated_mode when resume from S4.

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>