drm/fb-helper: Explain unload sequence a bit better

While reviewing Chris' patches to properly cancel our async workers I
checked that this happens after the fbdev is already unregistered.
That's the case, but I found a small gap in our docs, fill that in.

Note that I don't explain what release_fbi does, because that function
will disappear in the next patch ...

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170207141050.1129-1-daniel.vetter@ffwll.ch