thinkpad-acpi: fix issuing duplicated key events for brightness up/down

The tp_features.bright_acpimode will not be set correctly for brightness
control because ACPI_VIDEO_HID will not be located in ACPI. As a result,
a duplicated key event will always be sent. acpi_video_backlight_support()
is sufficient to detect standard ACPI brightness control.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>