drm/i915: Use true PPGTT in Gen8+ when execlists are enabled

In Gen8+, full ppgtt needs execlist, otherwise the ctx switch can hang.

Also remove the current restriction, a user should be able to explicitly set
ppgtt=2.

Note, this patch considers that execlist support has been enabled by
default on Gen8.

v2: Remove non-default restriction and clarify commit message (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
[danvet: s/comment/commit message/ in the commit message since that's
what Michel meant as per our irc discussion.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -102,8 +102,6 @@ static int sanitize_enable_ppgtt(struct drm_device *dev, int enable_ppgtt)
 
 	has_aliasing_ppgtt = INTEL_INFO(dev)->gen >= 6;
 	has_full_ppgtt = INTEL_INFO(dev)->gen >= 7;
-	if (IS_GEN8(dev))
-		has_full_ppgtt = false; /* XXX why? */
 
 	/*
 	 * We don't allow disabling PPGTT for gen9+ as it's a requirement for
@@ -134,6 +132,9 @@ static int sanitize_enable_ppgtt(struct drm_device *dev, int enable_ppgtt)
 		return 0;
 	}
 
+	if (INTEL_INFO(dev)->gen >= 8 && i915.enable_execlists)
+		return 2;
+	else
 		return has_aliasing_ppgtt ? 1 : 0;
 }