drm/i915: Use PIPE_CONTROL for flushing on gen6+.

v2 by danvet: Use a new flag to flush the render target cache on gen6+
(hw reuses the old write flush bit), as suggested by Ben Widawsdy.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
[danvet: this seems to fix cairo-perf-trace hangs on my snb]
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>