compositor: finish frame if redraw fails

compositor: finish frame if redraw fails

If we are about to finish a frame, but a redraw is pending and we let the
compositor redraw, we need to check for errors. If the redraw fails and
the backend cannot schedule a page-flip, we need to finish the frame,
anyway.

All backends except DRM use a timer to schedule frames. Hence, they cannot
fail. But for DRM, we need to be able to handle drmModePageFlip() failures
in case access got revoked.

This fixes a bug where logind+drm caused keyboard input to be missed as we
didn't reenable it after a failed page-flip during deactivation.