drm/amd/display: verify stream link before link test

[Why]
DP1.2 LL CTS test failure.

[How]
The failure is caused by not verify stream link is equal
to link, only check stream and link is not null.

Signed-off-by: Jing Zhou <Jing.Zhou@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>