drm/i915: Drop now misleading DDI comment from dp_link_down

Since

commit 2e82a720
Author: Imre Deak <imre.deak@intel.com>
Date:   Fri Jan 17 15:46:43 2014 +0200

    drm/i915: don't disable DP port after a failed link training

and

commit 5d6a1116
Author: Imre Deak <imre.deak@intel.com>
Date:   Thu Jan 16 18:35:57 2014 +0200

    drm/i915: don't disable the DP port if the link is lost

we no longer call intel_dp_link_down from generic DP code, but only
from the !HAS_DDI dp encoder functions. hsw/bdw have their own encoder
disabling callback in intel_ddi.c.

Hence the early return is no longer needed and the big comment just
confusing, so let's rip it out. To ensure what we don't accidentally
use this again on ddi encoders add a WARN_ON instead.

Spotted while reading through intel_dp.c

Cc: Imre Deak <imre.deak@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dp.c

@@ -3048,22 +3048,7 @@ intel_dp_link_down(struct intel_dp *intel_dp)
 		to_intel_crtc(intel_dig_port->base.base.crtc);
 	uint32_t DP = intel_dp->DP;
 
-	/*
-	 * DDI code has a strict mode set sequence and we should try to respect
-	 * it, otherwise we might hang the machine in many different ways. So we
-	 * really should be disabling the port only on a complete crtc_disable
-	 * sequence. This function is just called under two conditions on DDI
-	 * code:
-	 * - Link train failed while doing crtc_enable, and on this case we
-	 *   really should respect the mode set sequence and wait for a
-	 *   crtc_disable.
-	 * - Someone turned the monitor off and intel_dp_check_link_status
-	 *   called us. We don't need to disable the whole port on this case, so
-	 *   when someone turns the monitor on again,
-	 *   intel_ddi_prepare_link_retrain will take care of redoing the link
-	 *   train.
-	 */
-	if (HAS_DDI(dev))
+	if (WARN_ON(HAS_DDI(dev)))
 		return;
 
 	if (WARN_ON((I915_READ(intel_dp->output_reg) & DP_PORT_EN) == 0))