drm/imx: Remove unused field imx_drm_device.pipes

This has been unused since commit 44b460cf ("drm: imx: remove struct
imx_drm_crtc and imx_drm_crtc_helper_funcs")

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