drm/imx: store internal bus configuration in crtc state

The internal bus configuration is imx-drm specific crtc state. Store it
in imx_crtc_state and let the encoder atomic_check callbacks determine
bus_flags, bus_format and the sync pins, possibly taking into account
the mode and the connector display info.
The custom imx_drm_encoder structure can be replaced again with
drm_encoder.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>