drm/amd/display: Fix eDP panel light-up

 Power-down panel on disale only if sink exists
 otherwise it fails edid read during init sequence

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>