platform/chrome: chromeos_privacy_screen: Check ACPI_COMPANION()

platform/chrome: chromeos_privacy_screen: Check ACPI_COMPANION()

Every platform driver can be forced to match a device that doesn't match
its list of device IDs because of device_match_driver_override(), so
platform drivers that rely on the existence of a device's ACPI companion
object need to verify its presence.

Accordingly, add a requisite ACPI_COMPANION() check against NULL to the
chromeos_privacy_screen driver.

Fixes: d3c2872ae323 ("platform/chrome: Convert ChromeOS privacy-screen driver to platform")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/3357444.5fSG56mABF@rafael.j.wysocki
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>