drm/vc4: drv: Remove unused variable

The commit dcda7c28 ("drm/vc4: kms: Add functions to create the state
objects") removed the last users of the vc4 variable, but didn't remove
that variable resulting in a warning.

Fixes: dcda7c28

 ("drm/vc4: kms: Add functions to create the state objects")
Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102162908.1436567-1-maxime@cerno.tech