NULL pointer fixes

["Limonciello, Mario" <mario.limonciello@xxxxxxx>]

Hi,

The following commit landed in 6.4 that helps some occasional NULL 
pointer dereferences when setting up MST devices, particularly on 
suspend/resume cycles.

54d217406afe ("drm: use mgr->dev in drm_dbg_kms in 
drm_dp_add_payload_part2")

Can you please take this to 6.1.y and 6.3.y as well?  The NULL pointer 
de-reference has been reported on both.

Thanks,