gma500: gtt based hardware scrolling console

Add support for GTT based scrolling. Instead of pushing bits around we simply
use the GTT to change the mappings. This provides us with a very fast way to
scroll the display providing we have enough memory to allocate on 4K line
boundaries. In practice this seems to be the case except for very big displays
such as HDMI, and the usual configurations are netbooks/tablets.

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