gma500: silence gcc warnings in mid_get_vbt_data()

Add a cast here to silence a Gcc warning.
drivers/gpu/drm/gma500/mid_bios.c:214:28: warning:
  cast from pointer to integer of different size [-Wpointer-to-int-cast]

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>