drm/amd/display: fix the broken logic in dc_link.c

Add missing braces.

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>