drm/lease: Drop recursive leads checks

We disallow subleasing, so no point checking whether the master holds
all the leases - it will.

Spotted while typing exhaustive igt coverage for all these corner
cases.

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@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190228144910.26488-3-daniel.vetter@ffwll.ch