i965: Create a helper function for emitting PIPE_CONTROL flushes.

i965: Create a helper function for emitting PIPE_CONTROL flushes.

These days, we need to emit PIPE_CONTROL flushes all over the place.
Being able to do that via a single function call seems convenient.

Broadwell will also increase the length of these packets by 1; with the
refactoring, we should have to do this in substantially fewer places.

v2: Add back forgotten intel_emit_post_sync_nonzero_flush (caught by
    Eric Anholt).  Drop unlikely() from BLT_RING check.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>