drm/imx: merge imx-drm-core and ipuv3-crtc in one module

While it is possible to hook other CRTC implementations into imx-drm
in practice there are none yet and the option to disable ipuv3-crtc
support has been hidden for a long time.

Now that the imx-drm-core has learned to deal with some of the
specifics of IPUv3 there is a cyclic dependency between both parts.
To get rid of this and to decimate the Kconfig maze a bit, simply
merge both parts into one module.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>