drm/i915: suppress atomic commit error message under gvt-g env

under virtualization enviroment, it is possible guest update pipe
registers across vblank intervals due to overhead of mmio traps or vm
schedule out. However, it is safe since those pipe update happen in
virual registers and will not be committed to hardware. suppress that
atomic commit error message under virtualization case to avoid
confusing user.

v2: per ville's comment: return early and against Maarten's patch
v3: coding style clean

Signed-off-by: Bing Niu <bing.niu@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1489004043-15449-1-git-send-email-bing.niu@intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_sprite.c

@@ -161,6 +161,7 @@ void intel_pipe_update_end(struct intel_crtc *crtc, struct intel_flip_work *work
 	int scanline_end = intel_get_crtc_scanline(crtc);
 	u32 end_vbl_count = intel_crtc_get_vblank_counter(crtc);
 	ktime_t end_vbl_time = ktime_get();
+	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 
 	if (work) {
 		work->flip_queued_vblank = end_vbl_count;
@@ -186,6 +187,9 @@ void intel_pipe_update_end(struct intel_crtc *crtc, struct intel_flip_work *work
 
 	local_irq_enable();
 
+	if (intel_vgpu_active(dev_priv))
+		return;
+
 	if (crtc->debug.start_vbl_count &&
 	    crtc->debug.start_vbl_count != end_vbl_count) {
 		DRM_ERROR("Atomic update failure on pipe %c (start=%u end=%u) time %lld us, min %d, max %d, scanline start %d, end %d\n",