drm/i915: unlock on error in i915_ppgtt_info()

We need to call intel_runtime_pm_put() and mutex_unlock() before
returning.

Fixes: 7cb5dff8

 ('drm/i915: fix task reference leak in i915_debugfs.c')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>