ACPI / video: Fix acpi_video _register vs _unregister_backlight race

It is possible for a native backlight driver to load while
acpi_video_register is running, which may lead to
acpi_video_unregister_backlight being called while acpi_video_register
is running and the 2 racing against eachother.

The register_count variable protects against this, but not in a thread
safe manner, this commit adds locking to make this thread safe.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>