drm/i915: Only refcount ppgtt if it actually is one

This essentially unbreaks non-ppgtt operation where we'd scribble over
random memory.

While at it give the vm_to_ppgtt function a proper prefix and make it
a bit more paranoid.

Reviewed-by: Michel Thierry <michel.thierry@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 a5715fa..48d8f4a 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -2187,7 +2187,8 @@
 	if (!vma)
 		vma = __i915_gem_vma_create(obj, vm);
 
-	i915_ppgtt_get(vm_to_ppgtt(vm));
+	if (!i915_is_ggtt(vm))
+		i915_ppgtt_get(i915_vm_to_ppgtt(vm));
 
 	return vma;
 }