i965: Record mipmap resolver for unmapping

i965: Record mipmap resolver for unmapping

When mapping a region of the mipmap_tree, record which complementary
method to use to unmap it afterwards. By doing so we can avoid
duplicating the decision tree used when mapping and thereby eliminate
trivial errors that can be introduced if the two if-chains become out of
sync.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Scott D Phillips <scott.d.phillips@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>