drm/vc4: Use the helper to retrieve vc4_dev when needed

We have a helper to retrieve the vc4_dev structure from the drm_device one
when needed, so let's use it consistently.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-4-maxime@cerno.tech