apple-gmux: Stop using acpi_video_dmi_demote_vendor()

apple-gmux: Stop using acpi_video_dmi_demote_vendor()

acpi_video_dmi_demote_vendor() is going away as part of the cleanup of
the code for determinging which backlight class driver(s) to register.

The call to acpi_video_dmi_demote_vendor() was meant to undo the call to
acpi_video_dmi_promote_vendor() when the gmux device is removed, this is
questionable though since the promote call sets a flag, not a counter, so
the demote call may undo a promoto done elsewhere. Moreover in practice
this is a nop since the gmux device is never removed, and the flag is only
checked when acpi/video.ko gets loaded, so even if the user manually
removes apple-gmux the demote call is still a nop as video.ko will already
have loaded by this time.

Also note that none of the other users of acpi_video_dmi_promote_vendor()
use acpi_video_dmi_demote_vendor().

If we ever encounter a system with a gmux where the acpi-video interface
should be used, then the proper fix would be to dmi-blacklist the gmux
driver on that system.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>