drm/sun4i: backend: Use explicit fourcc helpers for packed YUV422 check

Checking for the number of planes is not sufficient to en ensure that
the format is a packed YUV422.

Use explicit fourcc helpers for the check instead.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190118145133.21281-4-paul.kocialkowski@bootlin.com