i915/display/intel_ddi: Reduce severity of failed FEC enabling

i915/display/intel_ddi: Reduce severity of failed FEC enabling

During some IGT tests (e.g. xe_pm@s2idle-exec-after, xe_pm@s2idle-mocs)
sink disconnects across suspend/resume, reconnecting later during resume
at some point. Hence during resume, where the driver is restoring the
pre-suspend mode, all the AUX transfers to the sink are expected to
fail.

Switch error message to KMS debug message of failed FEC enabling.

Signed-off-by: Michał Grzelak <michal.grzelak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20251117082046.4190705-1-michal.grzelak@intel.com