drm/vc4: txp: Remove duplicate regset

There's already a regset in the vc4_crtc structure so there's no need to
duplicate it in vc4_txp.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220711173939.1132294-51-maxime@cerno.tech