drm/i915: eDP link training optimization

This is a first of series patches that optimize DP link
training. The first patch is for eDP only where we reuse
the previously trained link training values from cache
i.e. voltage swing and pre-emphasis levels.

In case we are not able to train the link by reusing
the known values, the link training parameters are set
to zero and training is restarted.

V2:
- flag that indicates if DP link is trained and valid
  renamed from 'link_trained' to 'train_set_valid'
- removed routine 'intel_dp_reuse_link_train'

V3:
- rebased against the latest drm-intel-nightly

V4:
- removed HPD long pulse handling for eDP case to clear the
  flag that indicates to reuse the current link training
  parameters. (based on Sivakumar's comment)

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
[danvet: s/DP/eDP/ in subject to make scope clear.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>