drm/i915/dp: Remove ->mode_set callback

With all the preceding refactoring the dp mode_set callback only
computes a bit of state (all derived from the pipe config) and also
writes the eld. As long as we do that before we enable the audio bit
or depend upon the correct value in intel_dp->DP we'll be fine.

No other hw state is touched.

We therefore only need to check that clearing intel_dp->DP is save.
Which it is since when we re-enable we already mask out all the bits
the link training code sets. And we need to keep on doing that so that
the re-train loop walking over pre-emph/voltage-swing values still
works properly.

Reviewed-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>