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

Fix those compiler warnings, which indeed point to a bug:
drivers/char/agp/parisc-agp.c:228: warning: initialization from incompatible pointer type
drivers/char/agp/parisc-agp.c:201: warning: 'parisc_agp_page_mask_memory' defined but not used

Signed-off-by: Helge Deller <deller@gmx.de>