drm/amd/display: Skip wbscl_set_scaler_filter if filter is null

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Skip wbscl_set_scaler_filter if filter is null Callers can pass null in filter (i.e. from returned from the function wbscl_get_filter_coeffs_16p) and a null check is added to ensure that is not the case. This fixes 4 NULL_RETURNS issues reported by Coverity.