gma500: Add a gtt allocator

At the moment we don't do any page backing for the GTT so only the stolen
area pages will actually work. That is fine for our initial framebuffer and
a bit of testing but will need resolution (including alternate mmap methods
and the like for s/g areas) eventually.

Rather than use some of the overcomplex stuff in the DRM we use the existing
Linux resource allocators to hand out framebuffers and the like. This also has
the nice result that /proc/iomem shows the allocations.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>