drm/amd/display: Add Error Code for Dml Validation Failure

Any invalid mode from hw perspective should be given corresponding
error code, otherwise it leads to confusing warning message
"[drm] Mode Validation Warning: Validation OK failed validation."

Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>