gma500: The MID devices have the register offset different

This is another small step towards getting Moorestown/Oaktrail support to
work but for Moorestown at least we still need to sort out GEM backed base
framebuffer, which means figuring out why GEM explodes early on at the
moment.

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