drm/i915/skl: tell the user about pre-production hardware

We just removed the implementation for all the pre-production
workarounds, so now tell the user that we expect his machine to not
work properly. Also convert this to DRM_ERROR so we can more easily
spot these problems in bug reports and CI/QA runs.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474891672-23414-2-git-send-email-jani.nikula@intel.com