drm/sun4i: engine: Create an atomic_begin callback

We have to implement some display engine specific behaviours in
atomic_begin. Let's add a function for that.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/44110951ae0cc13767fefc7fc1d9e2ec782d0a40.1516613040.git-series.maxime.ripard@free-electrons.com