This implements most of the remaining u_threaded_context support.  Most
of the heavy lifting was done in the previous patches which fixed things
up for the new thread safety requirements.  Only a few things remain.

u_threaded_context support can be disabled via an environment variable:

   GALLIUM_THREAD=0

On Felix's Tigerlake with the GPU at fixed frequency, enabling
u_threaded_context improves performance of several games:

   - Civilization VI: +17%
   - Shadow of Mordor: +6%
   - Bioshock Infinite +6%
   - Xonotic: +6%

Various microbenchmarks improve substantially as well:

   - GfxBench5 gl_driver2: +58%
   - SynMark2 OglBatch6: +54%
   - Piglit drawoverhead: +25%

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <mesa/mesa!8964>