drm/sun4i: backend: Document the engine operations

Our operations were missing some documentation to explain what was expected
from them.

Let's make that clearer.

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/fdcd8ec3ae9ecd73ef089ede5218d3a41b49be05.1516613040.git-series.maxime.ripard@free-electrons.com