drm: add missing declaration to drm_blend.h

drm_blend.h is missing declaration for 'struct drm_plane'. Add it.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1494938085-21805-1-git-send-email-tomi.valkeinen@ti.com

include/drm/drm_blend.h

@@ -28,6 +28,7 @@
 
 struct drm_device;
 struct drm_atomic_state;
+struct drm_plane;
 
 /*
  * Rotation property bits. DRM_ROTATE_<degrees> rotates the image by the