drm/i915: Tune down WARN about incorrect VBT TC legacy flag

Looks like VBT contains again the wrong information about a port's TypeC
legacy vs. DP-alt/TBT-alt type. There is no further issues after we
notice this and fix it up, so tune down the WARN to be a a DRM_ERROR.

This also avoids CI tainting the kernel and stopping the test run.

v2:
- Update also code coment accordingly. (Jani)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110578


Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190502101754.29219-1-imre.deak@intel.com