drm/i915: untangle page flip completion

When a new page flip is requested, we need to both queue an unpin for
the current framebuffer, and also increment the flip pending count on
the newly submitted buffer.

At flip finish time, we need to unpin the old fb and decrement the flip
pending count on the new buffer.

The old code was conflating the two, and led to hangs when new direct
rendered apps were started, replacing the existing frame buffer.  This
patch splits out the buffers and prevents the hangs.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Eric Anholt <eric@anholt.net>