acpi-video-detect: Make acpi_video_get_capabilities a private function

acpi_video_get_capabilities() is only used inside video_detect.c so make
it static. While at it also remove the prototype for the non existent
acpi_video_display_switch_support function from acpi.h

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>