drm/i915/fbdev: Limit the global async-domain synchronization

During cleanup we have to synchronise with the async task we are using
to initialise and register our fbdev. Currently, we are using a full
synchronisation on the global domain, but we can restrict this to just
synchronising up to our task if we remember our cookie.

Whilst there, streamline the function parameters.

v2: async_synchronize_cookie() takes an exclusive upper bound, to
synchronize with our task we have to pass in the next cookie.
v3: Drop premature disregarding of the active cookie (we need to wait
until the task is complete before continuing in the teardown).
v4: Refactor waiting on async to incorporate a comment explaining why we
need the +1.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466497015-8509-2-git-send-email-chris@chris-wilson.co.uk