drm/i915: Always prefer CPU relocations with LLC

A follow-on to the update of the LLC coherency logic is that we can rely
on the LLC being coherent with the CS for rewriting batchbuffers
irrespective of their cache domain. (This should have no effect
currently as all the batch buffers are expected to be I915_CACHE_LLC and
so using the cpu relocation path anyway.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>