imx-drm: currently only IPUv3 is supported, make it mandatory

As long as only IPUv3 is supported in imx-drm, hide the separate
DRM_IMX_IPUV3 option and make DRM_IMX depend on IMX_IPUV3_CORE.

Reported-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>