drm/i915: force link training when requested by Sink

Compliance test 4.3.1.11 requires source to perform link training
always if the automated test requests for it. This patch
enforces this requirement.

Signed-off-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Shubhangi Shrivastava <shubhangi.shrivastava@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>