drm: rockchip: use crtc helper drm_crtc_from_index()

Function rockchip_crtc_from_pipe() does the exactly same thing as what
crtc helper drm_crtc_from_index() provides.  Use the helper to save
some code.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1483015290-16660-4-git-send-email-shawnguo@kernel.org