drm/amd/display: Correct the defined value for AMDGPU_DMUB_NOTIFICATION_MAX

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Correct the defined value for AMDGPU_DMUB_NOTIFICATION_MAX [Why & How] It actually exposes '6' types in enum dmub_notification_type. Not 5. Using smaller number to create array dmub_callback & dmub_thread_offload has potential to access item out of array bound. Fix it.