modetest: Fix pitches, somewhat

libkms only has the xrgb8888 format, so we're overallocating the bo by
quite a lot in some cases. But we still need to get the pitch from the
libkms since it's the driver that decides how to align it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>