gallium: Add the ability to query a single pipeline statistics counter

Gallium historically has treated pipeline statistics queries as a single
query, PIPE_QUERY_PIPELINE_STATISTICS, which returns a block of 11
values.  This was originally patterned after the D3D1x API.  Much later,
Brian introduced an OpenGL extension that exposed these counters - but
it exposes 11 separate queries, each of which returns a single value.

Today, st/mesa simply queries all 11 values, and returns a single value.
While pipeline statistics counters aren't typically performance
critical, this is still not a great fit.  A D3D1x->GL translator might
request all 11 counters by creating 11 separate GL queries...which
Gallium would map to reads of all 11 values each time, resulting in a
total 121 counter reads.  That's not ideal.

This patch adds a new cap, PIPE_CAP_QUERY_PIPELINE_STATISTICS_SINGLE,
and corresponding query type PIPE_QUERY_PIPELINE_STATISTICS_SINGLE.
When calling create_query(), q->index should be set to one of the
PIPE_STAT_QUERY_* enums to select a counter.  Unlike the block query,
this returns the value in pipe_query_result::u64 (as it's a single
value) instead of the pipe_query_data_pipeline_statistics group.

We update st/mesa to expose ARB_pipeline_statistics_query if either
capability is set, preferring the new SINGLE variant when available.

Thanks to Roland, Ilia, and Marek for helping me sort this out.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/mesa/state_tracker/st_context.c

@@ -462,6 +462,8 @@ st_create_context_priv(struct gl_context *ctx, struct pipe_context *pipe,
       screen->get_param(screen, PIPE_CAP_TGSI_PACK_HALF_FLOAT);
    st->has_multi_draw_indirect =
       screen->get_param(screen, PIPE_CAP_MULTI_DRAW_INDIRECT);
+   st->has_single_pipe_stat =
+      screen->get_param(screen, PIPE_CAP_QUERY_PIPELINE_STATISTICS_SINGLE);
 
    st->has_hw_atomics =
       screen->get_shader_param(screen, PIPE_SHADER_FRAGMENT,