staging: udlfb: support for writing backup EDID to sysfs file

staging: udlfb: support for writing backup EDID to sysfs file

Support writing default/backup edid via /sys/class/graphics/fb?/edid

Writing a backup edid blob to the 'edid' file will cause udlfb to
attempt to re-read the edid from hardware.
If we still don't have a good edid, it will use the backup edid instead.
The intent is to 1) enable user-supplied edid
2) enable udev rules to backup the last known good edid
for a device, and then provide that edid as a backup for all future starts

Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>