drm/i915: Abstract PPGTT enabling

Since we've already set up a nice vtable to abstract other PPGTT
functions, also abstract the actual register programming to enable
things.

This function will probably need to change a bit as we implement real
processes.

v2: Resolve conflicts due to patch series reordering.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index bd92e7d..70d10de 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -927,7 +927,8 @@
 			ring->init(ring);
 
 		i915_gem_context_init(dev);
-		i915_gem_init_ppgtt(dev);
+		if (dev_priv->mm.aliasing_ppgtt)
+			dev_priv->mm.aliasing_ppgtt->enable(dev);
 
 		/*
 		 * It would make sense to re-init all the other hw state, at