drm/i915: make fbdev initialization asynchronous v2

This gets us out of our init code and out to userspace quite a bit
faster, but does open us up to some bugs given the state of our init
time locking.

v2: switch to async_schedule (Chris)
    check with lockdep, seems happy (Jesse)
    move hotplug enable flag set to fbdev_initial_config (Jesse)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Rebase on top of the dev_priv->enable_hotplug_processing
removal.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>