gma500: introduce the framebuffer support code

We support 2D acceleration on some devices but we try and do tricks with
the GTT as a starting point as this is far faster. The GTT logic could be
improved further but for most display sizes it already makes a pretty good
decision.

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