drm/vc4: txp: Remove vc4_dev txp pointer

There's no user for that pointer so let's just get rid of it.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-50-maxime@cerno.tech