drm/i915/dp: reduce missing TPS3 support errors to debug logging

drm/i915/dp: reduce missing TPS3 support errors to debug logging

Per spec, TPS3 support is mandatory for downstream devices that support
HBR2. We've therefore logged errors on HBR2 without TPS3 since

commit 1da7d7131c35cde83f1bab8ec732b57b69bef814
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Thu Sep 3 11:16:08 2015 +0300

    drm/i915: ignore link rate in TPS3 selection

However, it seems there are real world devices out there that just
aren't spec compliant, and still work at HBR2 using TPS2. So reduce the
error message to debug logging.

Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Cc: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92932
Fixes: 1da7d7131c35 ("drm/i915: ignore link rate in TPS3 selection")
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454667370-8001-2-git-send-email-jani.nikula@intel.com
(cherry picked from commit bfcef5d2135ea1200ac1ea44661619ab8785c9f0)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>