drm/i915: Fix VCS ring selection after uapi decoupling

This got broken in:

   commit de1add36


   Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
   Date:   Fri Jan 15 15:12:50 2016 +0000

       drm/i915: Decouple execbuf uAPI from internal implementation

BSD ring flags need to be shifted before they can be considered
indices into the ring array.

Reported by Zhipeng Gong.

v2: Simplify the code. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhipeng Gong <zhipeng.gong@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1453902069-31353-1-git-send-email-tvrtko.ursulin@linux.intel.com
Testcase: igt/gem_exec_basic # bdw-gt3