drm/amd/display: fix return value check for hdcp_work

max_caps might be 0, thus hdcp_work might be ZERO_SIZE_PTR

Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>