i965: Rename gen6_*_constants tracked state atoms to "push_constants".

i965: Rename gen6_*_constants tracked state atoms to "push_constants".

When reading the "brw_wm_constants" and "gen6_wm_constants" atoms
side-by-side, I initially failed to notice the crucial difference:
the Gen6 atoms are for Push Constants, while brw_wm_constants handles
Pull Constants.  (Gen4/5 Push Constants are handled by "brw_curbe.")

Renaming these should clarify the code and save me from constant
confusion over the fact that "gen6_wm_constants" isn't just a newer
version of "brw_wm_constants."

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>