st/mesa: Make an enum for pipeline statistics query result indices.

st/mesa: Make an enum for pipeline statistics query result indices.

Gallium handles pipeline statistics queries as a single query
(PIPE_QUERY_PIPELINE_STATISTICS) which returns a struct with 11 values.
Sometimes it's useful to refer to each of those values individually,
rather than as a group.  To avoid hardcoding numbers, we define a new
enum for each value.  Here, the name and enum value correspond to the
index in the struct pipe_query_data_pipeline_statistics result.

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