drm/i915: Encode fence specific waitqueue behaviour into the wait.flags

Use the wait_queue_entry.flags to denote the special fence behaviour
(flattening continuations along fence chains, and for propagating
errors) rather than trying to detect ordinary waiters by their
functions.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201216165850.25030-1-chris@chris-wilson.co.uk