gma500: enable GEM mmap

Support mapping of GEM objects. This ought to be a small plumbing change but
instead we have to cut and paste a pile of stuff into the driver. This
really wants to be handled *IN* GEM

You can now allocate, mmap and munmap GEM objects in the driver. You can't
yet map them into the GART or display them however.

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