drm: Update docs for legacy kms state

Point at the equivalent atomic state and explain that atomic drivers
shouldn't really depend upon legacy state.

Motivated by questions from Manasi about how this all is supposed to
work.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171108203007.12274-1-daniel.vetter@ffwll.ch

include/drm/drm_encoder.h

@@ -88,7 +88,6 @@ struct drm_encoder_funcs {
  * @head: list management
  * @base: base KMS object
  * @name: human readable name, can be overwritten by the driver
- * @crtc: currently bound CRTC
  * @bridge: bridge associated to the encoder
  * @funcs: control functions
  * @helper_private: mid-layer private data
@@ -166,6 +165,11 @@ struct drm_encoder {
 	 */
 	uint32_t possible_clones;
 
+	/**
+	 * @crtc: Currently bound CRTC, only really meaningful for non-atomic
+	 * drivers.  Atomic drivers should instead check
+	 * &drm_connector_state.crtc.
+	 */
 	struct drm_crtc *crtc;
 	struct drm_bridge *bridge;
 	const struct drm_encoder_funcs *funcs;