mesa/main/util: moving gallium u_mm to util, remove main/mm
Right now there are two copies of mm:
At some point they splitted, and from the commit message it was not clear why it was not possible to have only one copy at a common place.
Taking into account that was several years ago, Im assuming that it was not possible then.
This change would allow to have one copy of the same code, and also being able to use that code out of mesa/main or gallium, if needed.
This commit moves u_mm and removes mm, as u_mm has slightly more changes.