drm/amd/display: Fix incorrect HDCP caps for dongle

[Why]
Previously we used link signal type to get the caps. We should use the
sink signal type

[How]
Use sink signal type instead of link signal type

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>