drm/amd/display: Avoid a NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Avoid a NULL pointer dereference [WHY] Although unlikely drm_atomic_get_new_connector_state() or drm_atomic_get_old_connector_state() can return NULL. [HOW] Check returns before dereference. (cherry picked from commit 1e5e8d672fec9f2ab352be121be971877bff2af9)