Instead of having these functions sprinkled around the driver (and ending
with a duplicated tu6_compare_func for example), move everything to a
common header (using the previously unused tu_util.h).

Also applied some simplifications: using a cast when the HW enum matches
the VK enum, and using a lookup table when it makes sense (which is IMO
nicer than the switch case way).

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <!5538>