drm/imx: Remove encoders' ->prepare callbacks

The main task of imx encoders' ->prepare callbacks is to set bus_format,
bus_flags, di_vsync_pin and di_hsync_pin.  We may create a structure named
imx_encoder to cache them.  The atomic encoder callback ->disable may
replace ->prepare later, so let's remove ->prepare.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>