drm/i915/gt: Do not restore invalid RS state

Only restore valid resource streamer state from the context image, i.e.
avoid restoring if we know the image is invalid.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/446


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191229183153.3719869-4-chris@chris-wilson.co.uk


Cc: stable@vger.kernel.org
(cherry picked from commit ecfcd2da)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>