drm/ast: Fix black screen when getting out of suspend

With an AST2600, the screen is garbage when going out of suspend.
This is because color settings are lost, and not restored on resume.
Force the color settings on DPMS_ON, to make sure the settings are correct.

I didn't write this code, it comes from the out-of-tree aspeed driver v1.13
https://www.aspeedtech.com/support_driver/



Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Venkat Tadikonda <venkateswara.rao@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622124815.356035-1-jfalempe@redhat.com