drm/i915/dp: Flush any outstanding work to turn the VDD off

As we may kick off a delayed workqueue task to switch of the VDD lines, we
need to complete that task prior to turning off the panel (which itself
depends upon VDD being off).

v2: Don't cancel the outstanding work as this may trigger a deadlock

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>