drm/i915/dp: remove DPMS mode tracking from DP

We currently use this when a hot plug event is received, only checking
the link status and re-training if we had previously configured a link.
However if we want to preserve the DP configuration across both hot plug
and DPMS events (which we do for userspace apps that don't respond to
hot plug uevents), we need to unconditionally check the link and try to
bring it up on hot plug.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>