i965/miptree: Use miptree_map in map_blit functions

This struct contains all the data of interest. can_blit_slice() will use
it in the next patch to calculate the correct pitch.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 030b6efc)