drm/amd/display: keep domain24 power on if eDP not exist

This is w/a: we need to keep domain 24 power up in driver side, and let
dmubfw handle it for S0i3. For last display unplugged, if OTG in PG, no
interrupt call back coming.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Chris Park <chris.park@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>