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.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>