i965: Add a function to emit the MI_REPORT_PERF_COUNT packet.

i965: Add a function to emit the MI_REPORT_PERF_COUNT packet.

MI_REPORT_PERF_COUNT writes a snapshot of the Observability Architecture
counters to a buffer.  Exactly how it works varies between generations:
Ironlake requires two packets, Sandybridge has to use GGTT, and Ivybridge
and later use PPGTT.

v2: Assert that we didn't use more space than we reserved (suggested
    by Eric Anholt).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>