drm/i915/skl: Don't allow disabling ppgtt and execlists on gen9+

Running the driver without execlists and hence PPGTT (either aliasing or
full) isn't a supported configuration on gen9+.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 68a27b2..29a4486 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -43,7 +43,12 @@
 	if (IS_GEN8(dev))
 		has_full_ppgtt = false; /* XXX why? */
 
-	if (enable_ppgtt == 0 || !has_aliasing_ppgtt)
+	/*
+	 * We don't allow disabling PPGTT for gen9+ as it's a requirement for
+	 * execlists, the sole mechanism available to submit work.
+	 */
+	if (INTEL_INFO(dev)->gen < 9 &&
+	    (enable_ppgtt == 0 || !has_aliasing_ppgtt))
 		return 0;
 
 	if (enable_ppgtt == 1)