drm/i915: Always enable execlists on BDW for vgpu

Broadwell hardware supports both ring buffer mode and execlist mode.
When i915 runs inside a VM with Intel GVT-g, we allow execlist mode
only.

The main reason of EXECLIST only is that GVT-g does not support the
dynamic mode switch between ring buffer mode and execlist mode when
running multiple virtual machines.

v2:
- Adjust the position of vgpu check in sanitize function (Joonas)
- Add vgpu error check in context initialization. (Joonas, Daniel)

Signed-off-by: Zhiyuan Lv <zhiyuan.lv@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -236,6 +236,12 @@ int intel_sanitize_enable_execlists(struct drm_device *dev, int enable_execlists
 {
 	WARN_ON(i915.enable_ppgtt == -1);
 
+	/* On platforms with execlist available, vGPU will only
+	 * support execlist mode, no ring buffer mode.
+	 */
+	if (HAS_LOGICAL_RING_CONTEXTS(dev) && intel_vgpu_active(dev))
+		return 1;
+
 	if (INTEL_INFO(dev)->gen >= 9)
 		return 1;