[REGRESSION] NULL pointer dereference drm_dp_add_payload_part2

Hello,

54d217406afe250d7a768783baaa79a035f21d38 fixed an issue in drm_dp_add_payload_part2 that lead to a NULL pointer dereference in case state is NULL.

The change was (accidentally?) reverted in 5aa1dfcdf0a429e4941e2eef75b006a8c7a8ac49 and the problem reappeared.

The issue is rather spurious, but I've had it appear when unplugging a thunderbolt dock.

#regzbot introduced 5aa1dfcdf0a429e4941e2eef75b006a8c7a8ac49