drm/vc4: drv: Use managed drm_mode_config_init

Using drmm_mode_config_init instead of drm_mode_config_init allows us to
cleanup a bit the error path.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-2-maxime@cerno.tech