drm/vc4: txp: Reorder the variable assignments

The current order of variable assignments is unneccessarily complex,
let's make it simpler.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123-rpi-kunit-tests-v1-14-051a0bb60a16@cerno.tech