drm/i915: Put back lane_count into intel_dp and add link_rate too

drm/i915: Put back lane_count into intel_dp and add link_rate too

With MST there won't be a crtc assigned to the main link encoder, so
trying to dig up the pipe_config from there is a recipe for an oops.

Instead store the parameters (lane_count and link_rate) in the encoder,
and use those values during link training etc. Since those parameters
are now assigned only when the link is actually enabled,
.compute_config() won't clobber them as it did before.

Hardware state readout is still bonkers though as we don't transfer the
link parameters from pipe_config intel_dp. We should do that during
encoder sanitation. But since we don't even do a proper job of reading
out the main link encoder state for MST there's littel point in
worrying about this now.

Fixes a regression with MST caused by:
 commit 90a6b7b052b1aa17fbb98b049e9c8b7f729c35a7
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Mon Jul 6 16:39:15 2015 +0300

    drm/i915: Move intel_dp->lane_count into pipe_config

v2: Different apporoach that should keep intel_dp_check_mst_status()
    somewhat less oopsy

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>