agp: parisc-agp.c - use correct page_mask function

This commit:

Commit 2a4ceb6d
Author: David Woodhouse <David.Woodhouse@intel.com>
Date:   Mon Jul 27 10:27:29 2009 +0100
     agp: Switch mask_memory() method to take address argument again, not page

broke the parisc AGP driver (again). This patch fixes it.

Signed-off-by: Helge Deller <deller@gmx.de>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>