drm/i915: Drop now misleading DDI comment from dp_link_down

drm/i915: Drop now misleading DDI comment from dp_link_down

Since

commit 2e82a7203182d0883d0f9450d40ad6e1c6578ad9
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 5d6a1116c6475404e6505b708320f9579ae19acd
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>