drm/i915: Stop clearing i915_ggtt_view

As we now use a compact memcmp in i915_vma_compare(), we can forgo
clearing the entire view and only set the precise parameters used in
this view.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170114002827.31315-4-chris@chris-wilson.co.uk