Revert and fix "drm/i915/dp: remove DPMS mode tracking from DP"

This reverts commit 885a5014.

We actually *do* need to track DPMS state so that on hotplug, we don't
retrain the link until DPMS is disabled.

However, that code had avery small bug -- it wouldn't set the
dpms_mode at mode set time, and so link retraining would not actually
occur on monitor hotplug until the monitor had gone through a DPMS
off/DPMS on cycle.

Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Andrew Lutomirski <luto@mit.edu>