This patch adds DP link training optimization by reusing the
previously trained values.

v2:
- rebase

V3:
- rebase

V4:
- when HPD long pulse is received, the flag is cleared
  that indicates if DP link training is required or not
  (based on Sivakumar's comment)
Signed-off-by: NMika Kahola <mika.kahola@intel.com>
Reviewed-by: NSivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>