drm/radeon: Fix kernel panic on shutdown

drm/radeon: Fix kernel panic on shutdown

Since commit 3e1cfc4ca5e8 ("drm/radeon: always apply pci shutdown
callbacks"), a Dell Latitude D600 laptop has crashed on shutdown. The
PCI Identification of the graphics adapter is "VGA compatible controller
[0300]: Advanced Micro Devices, Inc. [AMD/ATI] RV250/M9 GL [Mobility
FireGL 9000/Radeon 9000] [1002:4c66] (rev 01)".

Prior to commit 951a03461a0f ("drm/radeon: fix up dp aux tear down (v2)"),
I have no idea where the panic happened as the screen was blanked before
the crash.  Since that more recent change, the panic has been in routine
radeon_connector_unregister(), and has been shown to be due to a NULL
value in the ddc_bus member of struct drm_connector.

Bug: https://bugzilla.kernel.org/show_bug.cgi?id=178421
Fixes: 3e1cfc4ca5e8 ("drm/radeon: always apply pci shutdown callbacks")
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>