drm/fb_cma_helper: Add missing forward declaration

drm/fb_cma_helper: Add missing forward declaration

Add missing forward declaration for struct drm_plane and drm_plane_state,
which causes the following warning in the VC4 driver (can be replicated
by building using bcm2835_defconfig):

In file included from drivers/gpu/drm/vc4/vc4_drv.c:18:0:
include/drm/drm_fb_cma_helper.h:45:13: warning: ‘struct drm_plane_state’ declared inside parameter list will not be visible outside of this definition or declaration
      struct drm_plane_state *state);
             ^~~~~~~~~~~~~~~
include/drm/drm_fb_cma_helper.h:44:34: warning: ‘struct drm_plane’ declared inside parameter list will not be visible outside of this definition or declaration
 int drm_fb_cma_prepare_fb(struct drm_plane *plane,

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161115105529.3227-1-marex@denx.de