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