llvmpipe: enable PIPE_CAP_QUERY_PIPELINE_STATISTICS

llvmpipe: enable PIPE_CAP_QUERY_PIPELINE_STATISTICS

This was implemented since forever, but not enabled.
It passes all piglit tests except one, arb_pipeline_statistics_query-frag.
The reason is that the test (for drawing a 10x10 rect) expects between
100 and 150 pixel shader invocations. But since llvmpipe counts this with
4x4 granularity (and due to the rect being 2 tris) we end up with 224
invocations. I believe however what llvmpipe is doing violates neither the
spirit nor the letter of the spec (our fragment shader granularity really
is 4x4 pixels, albeit we will bail out early on 2x2 or 4x2 (the latter
if AVX is available) granularity), the spec allows to count additional
invocations due to implementation reasons.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>