drm/amd/display: Handle null 'stream_status' in 'planes_changed_for_existing_stream'

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Handle null 'stream_status' in 'planes_changed_for_existing_stream' This commit adds a null check for 'stream_status' in the function 'planes_changed_for_existing_stream'. Previously, the code assumed 'stream_status' could be null, but did not handle the case where it was actually null. This could lead to a null pointer dereference. Reported by smatch: drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_resource.c:3784 planes_changed_for_existing_stream() error: we previously assumed 'stream_status' could be null (see line 3774)