drm: rockchip: remove struct rockchip_crtc_funcs

With the vblank hooks in struct drm_crtc_funcs, we do not need to
maintain struct rockchip_crtc_funcs and the related registration
functions.  Remove them.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1486458995-31018-18-git-send-email-shawnguo@kernel.org