drm/i915: Don't clflush before release phys object

When we teardown the backing storage for the phys object, we copy from
the coherent contiguous block back to the shmemfs object, clflushing as
we go. Trying to clflush the invalid sg beforehand just oops and would
be redundant (due to it already being coherent, and clflushed
afterwards).

Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161223145804.6605-3-chris@chris-wilson.co.uk