gma500: Set the mapping mask

Some boards such as the Intel D2700MUD allow you to have over 4GB of RAM.
The GTT on the PVR based devices is 32bit however. Hugh Dickins points out
that we should therefore be setting the mapping gfp mask.

This is not the whole fix for the problem. Some further shmem patches will
be needed to deal with the corner cases.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>