drm/imx: Drop pipe field from struct imx_drm_crtc

drm/imx: Drop pipe field from struct imx_drm_crtc

Use the drm_crtc_index() helper to determine the pipe number of the CRTC
instead.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-5-git-send-email-boris.brezillon@free-electrons.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>