drm/imx: Remove last traces of struct imx_drm_crtc

When the definition of this struct was removed a forward declaration and an
unused struct member were still left around. Remove them because they serve
no purpose.

Fixes 44b460cf ("drm: imx: remove struct imx_drm_crtc and imx_drm_crtc_helper_funcs")

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>