drm/vc4: Use for_each_plane_in_state

We want to hide drm_atomic_stat internals a bit better.

Cc: Eric Anholt <eric@anholt.net>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-6-git-send-email-daniel.vetter@ffwll.ch