drm/gma500: Increase max resolution for mode setting

commit cbbd379aa43890f36da934f5af619d2fb8ec3d87 upstream.

By having a higher max resolution we can now set up a virtual
framebuffer that spans several monitors. 4096 should be ok since we're
gen 3 or higher and should be enough for most dual head setups.

Bugzilla:
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-modesetting/+bug/1169147
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>