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>