drm/i915/skl: Apply eDP WA only for gen < 9

The eDP WA to stop link train based on port type is for HSW/BDW, not
required for SKL+.
Suggested by Satheesh

v2: Simplified the check befoe stop_link_train. Suggested by Satheesh.

v3: stop_link_train need not be called from intel_enable_ddi for gen >= 9

Suggested-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Cc: Satheeshakrishna M <satheeshakrishna.m@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>