intel/perf: move the perf context into perf

intel/perf: move the perf context into perf

The "context" that is necessary to submit and process perf commands to
the hardware was previously present in the brw_context.perfquery
struct.  This commit moves it into perf and provides a more
understandable name.

The intention is for this struct to be private, when all methods that
access it are migrated into perf.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>