udlfb: add more comprehensive support for DPMS FB_BLANK_* modes

Fixes earlier problems where monitor would not return from blank

Test with any DisplayLink-based USB 2.0 graphics adapter
sudo nano /sys/class/graphics/fb?/blank
and write out single digit FB_BLANK_* code from include/linux/fb.h

Supports on (0), blank (1), suspend (2,3), powerdown (4)

Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>