drm/amd/display: Add null check in resource_log_pipe_topology_update

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Add null check in resource_log_pipe_topology_update

[WHY]

When switching from "Extend" to "Second Display Only" we sometimes

call resource_get_otg_master_for_stream on a stream for the eDP,

which is disconnected. This leads to a null pointer dereference.

[HOW]

Added a null check in dc_resource.c/resource_log_pipe_topology_update.