intel: Refactor selection of miptree tiling

intel: Refactor selection of miptree tiling

This patch (1) extracts from intel_miptree_create() the spaghetti logic
that selects the tiling format, (2) rewrites that spaghetti into a lucid
form, and (3) moves it to a new function, intel_miptree_choose_tiling().
No behavioral change.

As a bonus, it is now evident that the force_y_tiling parameter to
intel_miptree_create() does not really force Y tiling.

v2 (Ken): Rebase on top of Eric's untiled-for-larger-than-aperture
changes.  This required passing in the miptree.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>