drm/sun4i: backend: Move the coord function in the shared part

The function supposed to update a plane's coordinates is called in both
branches of our function. Let's move it out the if statement.

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/2cd57bcf13652109da7bd5bbe12fa1d29429f02f.1516617243.git-series.maxime.ripard@free-electrons.com