drm/i915: Refactor Broadwell PIPE_CONTROL emission into a helper.

We'll want to reuse this for a workaround.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Rmove now unused int.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>