drm/vc4: Use devm_drm_dev_alloc

We can simplify a bit the bind code, its error path and unbind by using
the managed devm_drm_dev_alloc function.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-5-maxime@cerno.tech