drm/i915: Remove the enabling of VS_TIMER_DISPATCH bit in MI MODE reg

This patch Removes the VS_TIMER_DISPATCH bit enable in MI MODE reg for
platforms > Gen6.
VS_TIMER_DISPATCH bit enable was earlier required as a part of
WA 'WaTimedSingleVertexDispatch', which is now applicable only to
platforms < Gen7.

v2: Enhancing the scope of the patch to full Gen7 (Chris)

v3: Modifying the WA condition to the cover the applicable platforms,
and adding the WA name in comments. (Ville)
Signed-off-by: NAkash Goel <akash.goel@intel.com>
Signed-off-by: NSourab Gupta <sourab.gupta@intel.com>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk> # ivb, hsw -Chris
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -566,7 +566,8 @@ static int init_render_ring(struct intel_ring_buffer *ring)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int ret = init_ring_common(ring);
 
-	if (INTEL_INFO(dev)->gen > 3)
+	/* WaTimedSingleVertexDispatch:cl,bw,ctg,elk,ilk,snb */
+	if (INTEL_INFO(dev)->gen >= 4 && INTEL_INFO(dev)->gen < 7)
 		I915_WRITE(MI_MODE, _MASKED_BIT_ENABLE(VS_TIMER_DISPATCH));
 
 	/* We need to disable the AsyncFlip performance optimisations in order