drm: gma500: Kill the GEM glue layer

The private gem_create_mmap_offset() function is now implemented in the
DRM core as drm_gem_create_mmap_offset(). Use it and kill the private
copy.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>