drm/vc4: kms: Add functions to create the state objects

In order to make the vc4_kms_load code and error path a bit easier to
read and extend, add functions to create state objects, and use managed
actions to cleanup if needed.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-6-maxime@cerno.tech