i915: Fix opregion notifications

opregion-based platforms will send ACPI video event 0x80 for a range of
notification types for legacy compatibility. This is interpreted as a
display switch event, which may not be appropriate in the circumstances.
When we receive such an event we should make sure that the platform is
genuinely requesting a display switch before passing that event through
to userspace.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Tested-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>