pointer_input: position cursor on the primary output after a placeholder

this improves the out of the box behavior which has regressed a bit over
time. since we default to kcfg_ActiveMouseScreen=true the active screen
by default follows the pointer. during early startup we position the
pointer on a Placeholder output, this output does eventually get
replaced by the real output(s). because of the update logic we'd
re-position the pointer on the closest real output, but that isn't
necessarily the intended primary output.

e.g. consider an eDP + HDMI setup depending on the geometries involved
the cursor may end up on the HDMI screen by default rather than the eDP
resulting in plasma-welcome opening on the HDMI output.

to mitigate this problem we now track whether the last output was a
placeholder and if so we instead try to position the pointer on the
current primary output