drm/sun4i: framebuffer: Add a custom atomic_check

In order to support normalized zpos, we need to call
drm_atomic_normalize_zpos in our driver's drm_mode_config_funcs'
atomic_check.

Let's duplicate the definition of drm_atomic_helper_check for now.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/67cb4ca9889e6bf29314db37127ff15eed279c53.1516617243.git-series.maxime.ripard@free-electrons.com