vgsp-disp: only post the channel event on a real FINISHED notifier write

The UPDATE path posted the channel completion event on every UPDATE,
including armed=0 updates where no notifier was written (r1665 journal:
armed=0 `-> none` still posts). Those events drive the DDX's NVKMS
FLIP_OCCURRED handler (nvidia_drv.so 0xaee70), which reaps the four
notifier slots of every layer and has a fixed-point retry when the
completed layer does not match the first DPY object's layer; with
non-flip updates raising the event, Xorg spun there at 100 %.

Gate the post on `wrote` (armed + notifier written + FINISHED). r1666:
the 0xaee70 spin is gone from perf; Xorg moved on to a bounded XAA
action-list scan (nvidia_drv.so 0x4dc9c0, nvXAAHandleActionList) plus
do_vfs_ioctl/schedule, i.e. the server main loop is no longer wedged in
the flip reaper.

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