drm/i915/dp: Restrict link retrain workaround to external monitors

Commit '3cf71bc9 ("drm/i915: Re-apply "Perform link quality check,
unconditionally during long pulse"")' applies a work around for sinks
that don't signal link loss. The work around does not need to have to be
that broad as the issue was seen with only one particular monitor; limit
this only for external displays as eDP features like PSR turn off the link
and the driver ends up retraining the link seeeing that link is not
synchronized.

Cc: Lyude Paul <lyude@redhat.com>
Cc: Jan-Marek Glogowski <glogow@fbihome.de>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
References: 3cf71bc9

 ("drm/i915: Re-apply "Perform link quality check, unconditionally during long pulse"")
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180927205735.16651-2-dhinakaran.pandiyan@intel.com