drm/atomic: add a drm_atomic_clean_old_fb helper.

This is useful for all the boilerplate code about cleaning old_fb.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447237751-9663-4-git-send-email-maarten.lankhorst@ubuntu.com