ACPI: video: Allow selecting NVidia-WMI-EC or Apple GMUX backlight from the cmdline

The patches adding NVidia-WMI-EC and Apple GMUX backlight detection
support to acpi_video_get_backlight_type(), forgot to update
acpi_video_parse_cmdline() to allow manually selecting these from
the commandline.

Add support for these to acpi_video_parse_cmdline().

Fixes: fe7aebb4 ("ACPI: video: Add Nvidia WMI EC brightness control detection (v3)")
Fixes: 21245df3 ("ACPI: video: Add Apple GMUX brightness control detection")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>