drm/i915: Register ACPI video even when not modesetting

The ACPI video driver defers registration to the i915 driver if the
system supports opregion-mediated backlight control. This registration
was only being performed in the KMS case. Ensure it's done even if we
don't have modesetting enabled.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>