gma500: Mask out bits not part of the page table base address

Otherwise we can't ioremap the gtt and the screen gets garbled.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>