drm/encoder: Fix possible_clones documentation

drm/encoder: Fix possible_clones documentation

We say that all of the bits in possible_clones must be set before
calling drm_encoder_init(). This isn't true though, since:

* The driver may not even have all of the encoder objects that could be
  used as clones initialized at that point
* possible_crtcs isn't used at all outside of userspace, so it's not
  actually needed to initialize it until drm_dev_register()

So, fix it.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913222704.8241-2-lyude@redhat.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>