Revert "drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES"

The core fix was applied in

commit a63b03e2
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Jan 6 10:29:35 2015 +0000

    mutex: Always clear owner field upon mutex_unlock()

(note the absence of stable@ tag)

so we can now revert our band-aid commit 226e5ae9 for -next.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>