drm/amd/display: add missing null check

[why]
There is a coding error for a missing null check for stream pointer when iterating through
pipe_ctx.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>