The apple-gmux driver only binds to old GMUX devices which have an
IORESOURCE_IO resource (using inb()/outb()) rather then memory-mapped
IO (IORESOURCE_MEM).

T2 MacBooks use the new style GMUX devices (with IORESOURCE_MEM access),
so these are not supported by the apple-gmux driver. This is not a problem
since they have working ACPI video backlight support.

But the apple_gmux_present() helper only checks if an ACPI device with
the "APP000B" HID is present, causing acpi_video_get_backlight_type()
to return acpi_backlight_apple_gmux disabling the acpi_video backlight
device.

Add a new apple_gmux_backlight_present() helper which checks that
the "APP000B" device actually is an old GMUX device with an IORESOURCE_IO
resource.

This fixes the acpi_video0 backlight no longer registering on T2 MacBooks.

Note people are working to add support for the new style GMUX to Linux:
https://github.com/kekrby/linux-t2/commits/wip/hybrid-graphics

Once this lands this patch should be reverted so that
acpi_video_get_backlight_type() also prefers the gmux on new style GMUX
MacBooks, but for now this is necessary to avoid regressing backlight
control on T2 Macs.

Fixes: 21245df3 ("ACPI: video: Add Apple GMUX brightness control detection")
Reported-and-tested-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>