intel: Reserve enough space to finish occlusion queries on Gen6.

intel: Reserve enough space to finish occlusion queries on Gen6.

After realizing that brw_finish_batch emitted some final PIPE_CONTROLs
to record occlusion queries, Chris noted that we probably hadn't
reserved enough space to actually emit them.

Reserving a full 60 bytes seems a bit harsh, since we only need that
much if occlusion queries are actually active.  Plus, 28 bytes would be
sufficient for Gen7, and 24 for Gen4-5.

We could optimize this in the future, but it doesn't seem too critical.

NOTE: This is a candidate for stable release branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=53311
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>