drm/vc4: dpi: Remove vc4_dev dpi pointer

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

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>