drm/i915/dp_link_training: Convert DRM_DEBUG_KMS to drm_dbg_kms

One instance of DRM_DEBUG_KMS was leftover in dp_link_training, convert
it to the new shiny.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210310214845.29021-2-sean@poorly.run