drm/sun4i: Constify static structs

A number of static variables are not modified and can be made const to
allow the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200804215337.54594-1-rikard.falkeborn@gmail.com