drm/i915: Clear the error PTE just once on finish

We do not need to continually clear our dedicated PTE for error capture
as it will be updated and invalidated to the next object. Only at the
end do we wish to be sure that the PTE doesn't point back to any buffer.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001194447.29910-3-chris@chris-wilson.co.uk