i965: Convert WM_STATE to genxml on gen4-5.

i965: Convert WM_STATE to genxml on gen4-5.

The code doesn't get exactly a lot simpler but at least it is in a single
place, and we delete more than we add.

Another good point is that you get rid of struct brw_wm_unit_state
which was a third mechanism for encoding GEN state. We used to have
GENXML, manual packing and these bitfield structs. Now we're down to
just GENXML and some manual packing. (Khristian)

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>