drm/imx: atomic phase 1: Use transitional atomic CRTC and plane helpers

Use the drm_plane_helper_update/disable() and drm_helper_crtc_mode_set()
transitional atomic helpers.  The crtc->mode_set_nofb callback is added
so that the primary plane is no longer tied to the CRTC.  Check/update
logics are separated to make sure crtc->mode_set_nofb and plane->atomic_update
are always successful.  Also, some necessary logics are tweaked for a smooth
transition.

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>