platform/x86: nvidia-wmi-ec-backlight: Use acpi_video_get_backlight_type()

Add an acpi_video_get_backlight_type() == acpi_backlight_nvidia_wmi_ec
check. This will make nvidia-wmi-ec-backlight properly honor the user
selecting a different backlight driver through the acpi_backlight=...
kernel commandline option.

Since the auto-detect code check for nvidia-wmi-ec-backlight in
drivers/acpi/video_detect.c already checks that the WMI advertised
brightness-source is the embedded controller, this new check makes it
unnecessary for nvidia_wmi_ec_backlight_probe() to check this itself.

Suggested-by: Daniel Dadap <ddadap@nvidia.com>
Reviewed-by: Daniel Dadap <ddadap@nvidia.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>