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