ACPI: video: Add Nvidia WMI EC brightness control detection (v3)

On some new laptop designs a new Nvidia specific WMI interface is present
which gives info about panel brightness control and may allow controlling
the brightness through this interface when the embedded controller is used
for brightness control.

When this WMI interface is present and indicates that the EC is used,
then this interface should be used for brightness control.

Changes in v2:
- Use the new shared nvidia-wmi-ec-backlight.h header for the
  WMI firmware API definitions
- ACPI_VIDEO can now be enabled on non X86 too,
  adjust the Kconfig changes to match this.

Changes in v3:
- Use WMI_BRIGHTNESS_GUID define

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>