fb: fix overlapping test off-by-one.

On my system with a radeon x2, the first GPU was not overlapping vesa
but the test decided it was.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>