drm/i915: Skip uncore lock on earlier gens

With gen < 6 we don't need to take uncore lock as we
don't have anything to protect from concurrent access.

v2: rebase and account for gen9 changes

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>