intel/perf: use temporary pointers to simplify access to perf state

intel/perf: use temporary pointers to simplify access to perf state

Most accesses to perf state were made through repeated dereferences of
brw_context members.  Prefering temporary variables of perf_ctx and
perf_cfg has the following advantages:

 - more concise implementation
 - easier refactor when moving subsequent methods to perf

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