ACPI / video: Revert "ACPI / video: driver must be registered before checking for keypresses"

On systems with an intel video opcode region, the completion used in the
patch this commit reverts will only complete if the i915 driver loads.

If for some reason the i915 driver never loads calls to
acpi_video_handles_brightness_key_presses() may be delayed indefinitely.

This reverts commit aecbd9b1 ("ACPI / video: driver must be registered
before checking for keypresses") fixing this.

This reintroduces a potential NULL pointer deref due to using an
uninitalized mutex, this is fixed differently in a follow-up patch.

Fixes: aecbd9b1

 (ACPI / video: driver must be registered before checking for keypresses)
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>