drm/i915: Add intel_bios_cleanup() function

Add an intel_bios_cleanup() function to act as counterpart of
intel_bios_init() and move the cleanup of vbt related resources there,
putting it in the same file as the allocation.

Changed in v2:
-While touching the code anyways, remove the unnecessary:
 if (dev_priv->vbt.child_dev) done before kfree(dev_priv->vbt.child_dev)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214082151.25015-1-hdegoede@redhat.com