drm: Update plane->fb also for page_flip

The legacy page_flip driver entry point is the only one left which
requires drivers to update plane->fb themselves. All the other entry
hooks will patch things up for the driver as needed since no one seems
to reliable get this right, see e.g. drm_mode_set_config_internal or
the plane->fb/old_fb handling in drm_mode_atomic_ioctl.

Therefore unify things, which allows us to ditch a TODO from
drm_atomic_helper_page_flip.

This should also help the atomic transition in i915 since we keep a
bit of legacy cruft only around because of this special behaviour in
->page_flip.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>