drm: move lease init after validation in drm_lease_create

Patch bd36d3ba fixed a deadlock in the
failure path of drm_lease_create. This made the partially initialized
lease object visible for a short window of time.

To avoid having the lessee state appear transiently, I've rearranged
the code so that the lessor fields are not filled in until the
parameters are all validated and the function will succeed.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171221065424.1304-1-keithp@keithp.com