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 <majun@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 8401bd36 ("drm/plane-helper: Add a drm_plane_helper_atomic_check() helper")
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.1+
Link: https://patchwork.freedesktop.org/patch/msgid/20221216030526.1335609-1-majun@amd.com