drm/i915: don't save/restore DSPARB on gen5+

Because the register does not exist in gen5+.

This patch solves "unclaimed register" messages on Haswell after
suspend/resume.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>