drm/i915: rip out intel_dp->dpms_mode

We now track the connector state in encoder->connectors_active, and
because the DP output can't be cloned, that is sufficient to track the
link state. Hence use this instead of adding yet another modeset state
variable with dubious semantics at driver load and resume time.

Also, connectors_active should only ever be set when the encoder is
linked to a crtc, hence convert that crtc test into a WARN.

v2: Rebase on top of struct intel_dp moving.

v3: The rebase accidentally killed the newly-introduced intel_dp->port
Noticed by Paulo Zanoni.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>