drm/imx: Remove imx_drm_crtc_id()

There is no one calling imx_drm_crtc_id() and it is just a simple
wrapper of drm_crtc_index() without doing any thing fancy - the
drivers may call drm_crtc_index() directly.  So, let's remove the
wrapper.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>