asus-wmi: Add egpu enable method

The X13 Flow laptops can utilise an external GPU. This requires
toggling an ACPI method which will first disable the internal
dGPU, and then enable the eGPU.

Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20210807023656.25020-4-luke@ljones.dev


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>