drm/i915: Fix missing unlock on error in i915_ppgtt_info()

Add the missing unlock before return from function i915_ppgtt_info()
in the error handling case.

Fixes: 1d2ac403(drm: Protect dev->filelist with its own mutex)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465861320-26221-1-git-send-email-weiyj_lk@163.com