i965: Implement threaded GL support.

i965: Implement threaded GL support.

Now i965 supports mesa_glthread=true like Gallium drivers do.

According to Markus (degasus), the Citra emulator now runs ~30% faster.
Emmanuel (linkmauve) also reported that the Dolphin emulator improved
by 2.8x on one game.  (Both of those still need to be added to drirc.)

An Intel Mesa CI run with mesa_glthread=true appears to be happy.

Bioshock Infinite's benchmark mode seems to be around 15-20% faster
on my Skylake GT4 at 1920x1080.

Tested-by: Markus Wick <markus@selfnet.de>
Tested-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>