drm/imx: atomic phase 3 step 1: Use atomic configuration

Replacing drm_crtc_helper_set_config() by drm_atomic_helper_set_config()
and converting the suspend/resume operations to atomic make us be able
to use atomic configurations.  All of these allow us to remove the
crtc_funcs->mode_set callback as it is no longer used.  Also, change
the plane_funcs->update/disable_plane callbacks from the transitional
version to the atomic version.  Furthermore, switching to the pure atomic
version of set_config callback means that we may implement CRTC/plane
atomic checks by using the new CRTC/plane states instead of the legacy
ones and we may remove the private ipu_crtc->enabled state which was left
there for the transitional atomic helpers in phase 1.  Page flip is also
switched to the atomic version.  Last, the legacy function
drm_helper_disable_unused_functions() is removed from ->load in order
not to confuse the atomic driver.

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>