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>

drivers/gpu/drm/i915/i915_drv.h

@@ -3167,7 +3167,6 @@ bool i915_gem_obj_is_pinned(struct drm_i915_gem_object *obj);
 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);
 }