drm/i915/bdw: Populate lrc with aliasing ppgtt if required

A previous commit broke aliasing PPGTT for lrc, resulting in a kernel oops
on boot. Add a check so that is full PPGTT is not in use the context is
populated with the aliasing PPGTT.

Issue: VIZ-4278
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -1452,12 +1452,17 @@ static int
 populate_lr_context(struct intel_context *ctx, struct drm_i915_gem_object *ctx_obj,
 		    struct intel_engine_cs *ring, struct intel_ringbuffer *ringbuf)
 {
+	struct drm_device *dev = ring->dev;
+	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_i915_gem_object *ring_obj = ringbuf->obj;
 	struct i915_hw_ppgtt *ppgtt = ctx->ppgtt;
 	struct page *page;
 	uint32_t *reg_state;
 	int ret;
 
+	if (!ppgtt)
+		ppgtt = dev_priv->mm.aliasing_ppgtt;
+
 	ret = i915_gem_object_set_to_cpu_domain(ctx_obj, true);
 	if (ret) {
 		DRM_DEBUG_DRIVER("Could not set to CPU domain\n");