drm/vc4: drv: Remove the DSI pointer in vc4_drv

That pointer isn't used anywhere, so there's no point in keeping it.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201203132543.861591-2-maxime@cerno.tech