Wake up DP sinks for DPCD read-based detection.

Wake up DP sinks for DPCD read-based detection.

When in DPMS sleep state the Apple mini DP->VGA adapter doesn't respond.
This led to no external display on boot.

v2: Avoid DPCD check and keep AUX awake for EDID read.

BUG=chrome-os-partner:17063
TEST=Straight DP and various adapters on mini-DP and DP platforms.

Change-Id: I90bea73006ae9c99464b0b5bc57f3803918313a8
Reviewed-on: https://gerrit.chromium.org/gerrit/42583
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: Stuart Abercrombie <sabercrombie@chromium.org>
Tested-by: Stuart Abercrombie <sabercrombie@chromium.org>