drm/amd/display: Add null pointer check in link_dpms

drm/amd/display: Add null pointer check in link_dpms

[why]
Check that the stream exists to add link->local_sink null pointer access
protection.

Reviewed-by: Harold Sun <harold.sun@amd.com>
Reviewed-by: Ethan Cheung <ethan.cheung@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>