[Intel] Centralize mipmap pitch computations.

[Intel] Centralize mipmap pitch computations.

mipmap pitches must account for the device alignment requirements, which
used to be fairly simple; just align to a 4-byte boundary. However, to allow
textures to be drawn to under TTM, they now need to be aligned to a 64-byte
boundary. Placing all of the alignment constraints in a single function
allows this new constraint to be applied uniformly.

There was some pitch constraining code in intel_miptree_create, but that was
modifying the pitch long after the miptree had been layed out, so it only
served to wreck the mipmap and cause rendering errors.