drm/i915: use VDD AUX override to make panel power sequencing look better

Rather than power cycling the panel when there are no bits to display,
use the VDD AUX bit to power the panel up just enough for DP AUX
transactions to work.  This prevents a bit of unnecessary ugliness as
mode sets occur on the panel.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>