drm/lease: Check for lessor outside of locks

The lessor is invariant over a lifetime of a lease, we don't have to
grab any locks for that. Speeds up the common case of not being a lease.

Cc: Keith Packard <keithp@keithp.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190228144910.26488-5-daniel.vetter@ffwll.ch