platform/x86: fix asus-wmi build error

Fix build error in asus_wmi.c when ASUS_WMI=y and ACPI_VIDEO=m
by preventing that combination.

  drivers/built-in.o: In function `asus_wmi_probe':
  asus-wmi.c:(.text+0x65ddb4): undefined reference to `acpi_video_unregister'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>