drm/i915: Demidlayer driver unloading

To complete the transition to manual control of load/unload, we need to
take over unloading from i915_pci_remove(). This allows us to correctly
order our unregister vs shutdown phases, which currently are inverted
due to the midlayer.

However, the unload sequence is still invalid as we shutdown the driver
with the last reference. Ideally, all we want to do is remove the
userspace access on device removal, deferring the cleanup to the
drm_dev_release() - breaking the reference cycles is then left as an
exercise for the reader.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773227-7994-7-git-send-email-chris@chris-wilson.co.uk