drm/i915/userptr: Keep spin_lock/unlock in the same block

Move the code around in order to acquire and release the spinlock in the
same function and in the same block. This keeps static analysers happy
and the reader sane.

Suggested-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>