drm/i915: Only dump dp_m2_n2 configuration when drrs is used

Otherwise it is just an useless empty line.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479397449-27085-1-git-send-email-tvrtko.ursulin@linux.intel.com