drm/amd/display: Send DISPLAY_OFF after power down on boot

[WHY]
update_clocks might not be called on headless adapters. This means
DISPLAY_OFF may not be sent in headless cases.

[HOW]
If hardware is powered down on boot because it is headless (mode set
does not happen on that adapter) also send DISPLAY_OFF notification.

Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>