sun4i/drm: backend: use mode_set engine callback

Newly introduced mode_set callback in engine structure is a much better
place for setting backend output size and interlace mode for following
reasons:
1. Aforementioned properties change only when mode changes, so it's
   enough to be set only once per mode set. Currently it's done whenever
   properties of primary plane are changed.
2. It's assumed that primary plane will always cover whole screen. While
   this is true most of the time, it's not always. Planes are universal.
   There is no reason to add artificial limitation to primary plane.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
[Samuel: drop unused 'interlaced' variable]
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424162633.12369-8-samuel@sholland.org