drm/udl: limit modes to the sku pixel limits.

Otherwise when X starts we commonly get a black screen scanning
out nothing, its wierd dpms on/off from userspace brings it back,

With this on F18, multi-seat works again with my 1920x1200 monitor
which is above the sku limit for the device I have.

Cc: stable@vger.kernel.org
Reviewed-by: Alex Deucher <alexander.deucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>