drm/sun4i: backend: Fix structure indentation

The sun4i_plane_desc structure was somehow indented to two tabulations
instead of one as we shoud do. Fix that.
Acked-by: NChen-Yu Tsai <wens@csie.org>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8a6714bddb865adfcfe2b792e406a2f10bb819bc.1516617243.git-series.maxime.ripard@free-electrons.com