drm/i915/dp: promote clock recovery failures to DRM_ERROR

If channel equalization succeeds, there's no indication something went
wrong in clock recovery (unless debug is enabled). We should shout about
the failures and fix them instead of hiding them under the carpet.

This has allowed bugs like [1] stay dormant for a long time.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=70117



Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>