gxt4500: use display information in info not in var for panning

We must not use any information in the passed var besides xoffset,
yoffset and vmode as otherwise applications might abuse it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>