backends/drm: fix pageflip events getting lost when outputs get turned off

When a display gets turned off while a pageflip event was still pending, the pending
state gets changed immediately, without waiting for the pageflip event to arrive first.
When this happens, activePending() returns false, and so the pageflip event gets discarded.
This commit changes the logic to only check for activePending() when the pageflip is for a
modeset, avoiding that issue.

BUG: 476340
BUG: 476341
BUG: 476342
BUG: 437520
BUG: 475146