intel/mi_builder: Drop the gen_ prefix

intel/mi_builder: Drop the gen_ prefix

mi_ is already a unique prefix in Mesa so the gen_ isn't really gaining
us anything except extra characters.  It's possible that MI_ may
conflict a tiny bit with GenXML but it doesn't seem to be a problem
today and we can deal with that in the future if it's ever an issue.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9393>