radeonfb: 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. Also use the
aligned fix.line_length and not the (possible) unaligned xres_virtual.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>