gma500: introduce the GTT and MMU handling logic

This fits alongside the GEM support to manage our resources on the card
itself. It's not actually clear we need to configure the MMU at all.
Further research is needed before removing it entirely. For now we suck it
in (slightly abused) from the old semi-free driver.

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