I have same problem on latest kernel 4.9, but I just found some solution here.
Monitor (PHILIPS 273V5HSB 1920 x 1080) have until now been connected to computer by HDMI cable. I just tried to change to use VGA cable instead, then everything work fine and X start without problems in full 1920 x 1080. On kernel 4.7 monitor start fine on HDMI cable, but not on kernel 4.8 and up. Brgds Torben Schou Jensen
