drm/amd/display: NULL pointer hang

[Why]
In dc_link_dp_set_test_pattern, we assume all pipes have a stream, which
can cause null pointer dereference.

[How]
Add a null pointer check before accessing stream.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>