drm/syncobj: add sync_file interaction. (v1.2)

This interface allows importing the fence from a sync_file into
an existing drm sync object, or exporting the fence attached to
an existing drm sync object into a new sync file object.

This should only be used to interact with sync files where necessary.

v1.1: fence put fixes (Chris), drop fence from ioctl names (Chris)
fixup for new fence replace API.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>