drm/i915/dp: Reset frl trained flag before restarting FRL training

For cases where DP has HDMI2.1 sink and FRL Link issues are detected,
reset the flag to state FRL trained status before restarting FRL
training.

Fixes: 9488a030

 ("drm/i915: Add support for enabling link status and recovery")
Cc: Swati Sharma <swati2.sharma@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com> (v2)
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011063447.904649-2-ankit.k.nautiyal@intel.com