drm/i915: implement WaIssueDummyWriteToWakeupFromRC6

Or at least our best understanding of it.

v2: Fixup commit message and put the wa name into the comment block.
And actually update the commit, too.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>