drm/i915: Move fb_bits updating later in atomic_commit

Currently it's part of prepare_fb, still in the first phase of
atomic_commit which might fail. Which means that we need to have some
heuristics in cleanup_fb to figure out whether things failed, or
whether we just clean up the old fbs.

That's fragile, and worse, once we start pipelining commits gets
confused: While the last commit is still getting cleanup up we already
hammer in the new one, and fb_bits aren't refcounted, resulting in
lost bits and WARN_ON galore. We could instead try to make cleanup_fb
more clever, but a simpler fix is to postpone the fb_bits tracking
past the point of no return, where we commit all the software state.

That also makes conceptually more sense, since fb_bits must be updated
synchronously from the ioctl (they track usage from userspace pov, not
from the hw pov), right before we're fully committed to the updated.

This fixes WARNING splats from track_fb with page_flip implemented
through atomic_commit.

Testcase: igt/kms_flip/flip-vs-rmfb
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465827229-1704-4-git-send-email-daniel.vetter@ffwll.ch