We were doubling up the offsets for the mipmap levels for CPU access.
Instead of reimplementing i945_miptree_layout_2d with 6 cube images
separated by qpitch, share that function and provide the level offsets
later.

Fixes piglit cubemap and fbo-cubemap.
(cherry picked from commit 7d8f0fc2
Conflict due to mt->pitch still being here resolved -- i945 layout
should set it.)