drm/i915: Fix struct_mutex deadlock due to merge fumble

drm/i915: Fix struct_mutex deadlock due to merge fumble

commit 056904d9a2f12f4f4a7c404229ce8b8e7a550f5f
Merge: c2b83266 84539b0
Author: Dave Airlie <airlied@redhat.com>
Date:   Mon Mar 9 19:58:30 2015 +1000

    Merge tag 'v4.0-rc3' into drm-next

managed to pick the wrong code to resolve the conflict and left us with
a mutex_lock(struct_mutex) without the mutex_unlock(struct_mutex) leading
to a deadlock. Fix the problem by recovering the correct code which doesn't
need the lock.

Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>