drm/fb_cma_helper: Add drm_fb_cma_prepare_fb() helper

Add new drm_fb_cma_prepare_fb() helper function extracted from the
imx-drm driver. This function checks if the plane has DMABUF attached
to it, extracts the exclusive fence from it and attaches it to the
plane state for the atomic helper to wait on it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114100732.3446-1-marex@denx.de
diff --git a/include/drm/drm_fb_cma_helper.h b/include/drm/drm_fb_cma_helper.h
index f313211..cc82c73 100644
--- a/include/drm/drm_fb_cma_helper.h
+++ b/include/drm/drm_fb_cma_helper.h
@@ -41,6 +41,9 @@
 struct drm_gem_cma_object *drm_fb_cma_get_gem_obj(struct drm_framebuffer *fb,
 	unsigned int plane);
 
+int drm_fb_cma_prepare_fb(struct drm_plane *plane,
+			  struct drm_plane_state *state);
+
 #ifdef CONFIG_DEBUG_FS
 struct seq_file;