drm/ast: Convert I2C code to managed cleanup

Release the I2C adapter as part of the DRM device cleanup. Remove
ast's dedicated helper for struct drm_connector_funcs.destroy.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206091125.29501-3-tzimmermann@suse.de