intel: Store miptree alignment units in the miptree

intel: Store miptree alignment units in the miptree

This allows us to replace all the calls to
intel_get_texture_alignment_unit() with a single call at miptree creation.

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