drm/i915: Fix VCS ring selection after uapi decoupling

This got broken in:

   commit de1add360522c876c25ef2bbbbab1c94bdb509ab
   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
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 2dc08ce..5cb57f6 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1401,6 +1401,7 @@
 			bsd_idx = gen8_dispatch_bsd_ring(dev_priv, file);
 		} else if (bsd_idx >= I915_EXEC_BSD_RING1 &&
 			   bsd_idx <= I915_EXEC_BSD_RING2) {
+			bsd_idx >>= I915_EXEC_BSD_SHIFT;
 			bsd_idx--;
 		} else {
 			DRM_DEBUG("execbuf with unknown bsd ring: %u\n",