drm/mgag200: Do not call drm_atomic_add_affected_planes()

There's no need to add planes to the atomic state. Remove the call
to drm_atomic_add_affected_planes() from mgag200.

On full modesets, the DRM helpers already add a CRTC's planes to the
atomic state; see drm_atomic_helper_check_modeset(). There's no reason
to call drm_atomic_add_affected_planes() unconditionally in the CRTC's
atomic_check() in mgag200. It's also too late, as the atomic_check()
of the added planes will not be called before the commit.
Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221011165136.469750-2-javierm@redhat.com

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -601,7 +601,7 @@ int mgag200_crtc_helper_atomic_check(struct drm_crtc *crtc, struct drm_atomic_st
 		}
 	}
 
-	return drm_atomic_add_affected_planes(new_state, crtc);
+	return 0;
 }
 
 void mgag200_crtc_helper_atomic_flush(struct drm_crtc *crtc, struct drm_atomic_state *old_state)