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.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_encoder.h

@@ -154,7 +154,7 @@ struct drm_encoder {
 	 * using drm_encoder_index() as the index into the bitfield. The driver
 	 * must set the bits for all &drm_encoder objects which can clone a
 	 * &drm_crtc together with this encoder before calling
-	 * drm_encoder_init(). Drivers should set the bit representing the
+	 * drm_dev_register(). Drivers should set the bit representing the
 	 * encoder itself, too. Cloning bits should be set such that when two
 	 * encoders can be used in a cloned configuration, they both should have
 	 * each another bits set.