drm/i915: use VDD AUX for panel power around detection and in prepare

Mode setting sequence specifies that we use VDD AUX for configuration
and detection, and early in the mode set sequence.  Only later (after
DP_A has started training) should we actually enable panel power.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[ickle: checkpatch.pl complaining about whitespace]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>