drm/i915: Refactor gen6_flush_pd()

As the gen6 page directory is written on binding and after every update,
the code ended up duplicated. Refactor the code into a single routine to
share the locking and serialisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191201140916.2128905-1-chris@chris-wilson.co.uk