i965: Move intel_region_get_tile_masks() to be a miptree function.

i965: Move intel_region_get_tile_masks() to be a miptree function.

All the consumers are doing it on a miptree.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>