drm/i915: fix task reference leak in i915_debugfs.c

Leak a task reference in i915_ppgtt_info(), add put_task_struct()
to fix it.

Introduced by

commit 1c60fef5


Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Dec 6 14:11:30 2013 -0800

    drm/i915: Dump all ppgtt

Signed-off-by: Geliang Tang <geliangtang@163.com>
[danvet: Add note provided by Jani about which commit introduced this
issue.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>