drm: imx: remove struct imx_drm_crtc and imx_drm_crtc_helper_funcs

With the vblank hooks in struct drm_crtc_funcs, we do not need to
maintain the CRTC specific vblank callbacks with struct
imx_drm_crtc_helper_funcs any more.  By moving the stuff that we
currently do in imx_drm_add_crtc(), like of_node setting and
drm_crtc_helper_add()/drm_crtc_init_with_planes() invoking, we can kill
things like struct imx_drm_crtc, imx_drm_crtc_helper_funcs and related
functions completely.

Functions ipu_enable_vblank() and ipu_disable_vblank() are moved around
without changes, only for saving the forward declarations.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: http://patchwork.freedesktop.org/patch/msgid/1486458995-31018-13-git-send-email-shawnguo@kernel.org