st/mesa: implement GL_AMD_performance_monitor

st/mesa: implement GL_AMD_performance_monitor

This is based on the original patch of Christoph Bumiller.

v2 (Samuel Pitoiset):
 - improve Gallium interface for this extension
 - rewrite some parts of the original code
 - fix compilation errors and piglit tests

v3:
 - only enable this extension when the underlying driver expose GPU counters
 - get rid of the ring buffer of queries

v4:
 - add a debug message when the maximum number of counters has been
   reached

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>