ACPI / video: Fix using an uninitialized mutex / list_head in acpi_video_handles_brightness_key_presses()

If acpi_video_handles_brightness_key_presses() was called before
acpi_video_register(), it would use the video_list mutex / list_head
uninitialized.

This patch fixes this by using DEFINE_MUTEX / LIST_HEAD when declaring
these, instead of initializing them runtime from acpi_video_register().

Fixes: 90b066b1 "ACPI / video: Add a acpi_video_handles_brightness_key_presses() helper"
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>