i965: Reserve batchbuffer space for a closing MI_REPORT_PERF_COUNT.

In order to use the Observability Architecture effectively, we'll need
to take snapshots of the OA counters via MI_REPORT_PERF_COUNT at the
start and end of each batch.

Experimentation reveals that we need to flush before and after each
MI_REPORT_PERF_COUNT to get working values.  For simplicitly, I chose to
use intel_batchbuffer_emit_mi_flush(), which unfortunately expands to
triple pipe controls on Sandybridge.

We may want to start computing per-generation reserved batch space to
avoid the insanity of Sandybridge's PIPE_CONTROL cost.  That said, much
of this cost existed before I rewrote the query object support to use
hardware contexts, so it's at least not entirely new.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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