drm/i915: Do not WARN_ON in i915_vm_to_ppgtt

According to Chris, use of i915_vm_to_ppgtt is visible in benchmark
unless WARN_ON is removed, so lets get rid of it.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a1f78f2..4ebd3ff 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -3167,7 +3167,6 @@
 static inline struct i915_hw_ppgtt *
 i915_vm_to_ppgtt(struct i915_address_space *vm)
 {
-	WARN_ON(i915_is_ggtt(vm));
 	return container_of(vm, struct i915_hw_ppgtt, base);
 }