drm/i915: Take rpm wakelock around debugfs/i915_gpu_info

drm/i915: Take rpm wakelock around debugfs/i915_gpu_info

Capturing GPU state requires the device to be awake in order to read
registers. Normally, this is taken along the error handler, but for the
direct debugfs access, we cannot make assumptions about the current
device state and so either need to wake it up, or abort.

Fixes: 5a4c6f1b1b2d ("drm/i915: The return of i915_gpu_info to debugfs")
Testcase: igt/pm_rpm/debugfs-read
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170328131407.14863-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>