drm/amd/display: Fixed hardware power down bypass during headless boot

[Why]
During headless boot, DIG may be on which causes HW/SW discrepancies.
To avoid this we power down hardware on boot if DIG is turned on. With
introduction of multiple eDP, hardware power down is being bypassed
under certain conditions.

[How]
Fixed hardware power down bypass, and ensured hardware will power down
if DIG is on and seamless boot is not enabled.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>