mesa: use simple mtx in core mesa

Results from x11perf -copywinwin10 on Eric's SKL:
   4.33338% ± 0.905054% (n=40)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Tested-by: Yogesh Marathe <yogesh.marathe@intel.com>
14 files changed