drm/amd/display: Check null pointers before using them

[WHAT & HOW]
dc_link is null checked previously in the same function, indicating it
might be null as reported by Coverity.

This fixes 1 FORWARD_NULL issue reported by Coverity.

Signed-off-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>