mesa: always support occlusion queries
Excerpt from ARB_occlusion_query.txt:
An implementation can either set QUERY_COUNTER_BITS_ARB to the
value 0, or to some number greater than or equal to n. If an
implementation returns 0 for QUERY_COUNTER_BITS_ARB, then the
occlusion queries will always return that zero samples passed the
occlusion test, and so an application should not use occlusion queries
on that implementation.
This looks more sane for drivers wanting desktop gl 1.5 without real hw support then just faking it.