drm/i915/lrc: Only set RS ctx enable in ctx control reg if there is a RS

The driver should only set the "RS context enable" bit in the context
image if we plan to use the resource streamer.

Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456393738-35608-1-git-send-email-michel.thierry@intel.com