agp: Add generic support for graphics dma remapping

New driver hooks for support graphics memory dma remapping
are introduced in this patch. It makes generic code can
tell if current device needs dma remapping, then call driver
provided interfaces for mapping and unmapping. Change has
also been made to handle scratch_page in remapping case.

Signed-off-by: Zhenyu Wang <zhenyu.z.wang@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>