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

commit 3a75885848996baab5276ff37ebf7295c3c753f0 upstream.

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.

Reviewed-by: Alex Deucher <alexander.deucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>