drm/i915/dmabuf: Acquire the backing storage outside of struct_mutex

Use the per-object mm.lock to allocate the backing storage (and hold a
reference to it across the dmabuf access) without resorting to
struct_mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161028125858.23563-15-chris@chris-wilson.co.uk