drm/i915: split fb allocation and initialization v2

If we use a stolen buffer, our probe callback shouldn't allocate a new
buffer; we should re-use the one from the BIOS instead if possible.

v2: fix locking (Jesse)

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>