When we're in a page-flip retry loop due to the FIFO being full
(drmModePageFlip() failing with EBUSY), we should not continue to try
when starting to power save, as that means we're blocking new frames,
which inself blocks input events due to them being compressed using the
frame clock.

We'd also hit an assert assuming we only try to page flip when not power
saving.

Thus, fake we flipped if we ended up reaching a power saving state while
retrying.

Fixes: GNOME/mutter#509

GNOME/mutter!506