Revert "drm/i915/execlists: Use a local lock for dfs_link access"

This reverts commit 27745e82 ("drm/i915/execlists: Use a local lock
for dfs_link access") as the struct_mutex was required to prevent
concurrent retiring and freeing, now restored in the previous patch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161128143649.4289-2-chris@chris-wilson.co.uk


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>