drm/i915: handle non-flip pending case when unpinning the scanout buffer

The first page flip queued will replace the current front buffer, which
should have a 0 pending flip count.  So at finish time we need to handle
that case (i.e. if the flip count is 0 *or* dec_and_test is 0 we need to
wake the waiters).

Also fix up an error path in the queue function and add some debug
output (only enabled with driver debugging).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>