drm/vc4: tests: Use custom plane state for mock

The current mock planes were just using the regular drm_plane_state,
while the driver expect struct vc4_plane_state that subclasses
drm_plane_state.

Hook the proper implementations of reset, duplicate_state, destroy and
atomic_check to create vc4_plane_state.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>