scrn->display is a property of the main screen really, and we don't
want to have the GPU screens use it for anything when picking modes
or a front buffer size.

This fixes a bug where when you plugged a display link device, it
would try and allocate a screen the same size as the current running
one (3360x1050 in this case), which was too big for the device. Avoid
doing this and just pick sizes based on whats plugged into this device.

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>