drm/imx: switch to universal planes

Use drm_universal_plane_init to create the planes, create the primary
plane first and use drm_crtc_init_with_planes to associate it with
the crtc.
This gets rid of the unused fallback primary plane previously created
by drm_crtc_init and fixes a NULL pointer dereference issue that can
be triggered by a modeset from userspace when fbdev helpers are
enabled [1].

[1] https://lkml.org/lkml/2015/11/4/107



Reported-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Liu Ying <Ying.Liu@freescale.com>