drm/plane-helper: Add the missing declaration of drm_atomic_state

Add the missing declaration of struct drm_atomic_state to fix the
compile error below:

error: 'struct drm_atomic_state' declared inside parameter
list will not be visible outside of this definition or declaration [-Werror]

Signed-off-by: Ma Jun <[e-mail]>
Reviewed-by: Thomas Zimmermann <[e-mail]>
Signed-off-by: Thomas Zimmermann <[e-mail]>
Fixes: 8401bd361f59 ("drm/plane-helper: Add a drm_plane_helper_atomic_check() helper")
Cc: Javier Martinez Canillas <[e-mail]>
Cc: Thomas Zimmermann <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Cc: Maxime Ripard <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Cc: <[e-mail]> # v6.1+
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

include/drm/drm_plane_helper.h

···
 
 #include <linux/types.h>
 
+struct drm_atomic_state;
 struct drm_crtc;
 struct drm_framebuffer;
 struct drm_modeset_acquire_ctx;