drm/atomic: Remove drm_atomic_helper_framebuffer_changed.

This function is now completely unused, zap it.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1481204729-9058-6-git-send-email-maarten.lankhorst@linux.intel.com