drm/nouveau: use drm_mm in preference to custom code doing the same thing

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nv50_display.c b/drivers/gpu/drm/nouveau/nv50_display.c
index 580a5d1..515edde 100644
--- a/drivers/gpu/drm/nouveau/nv50_display.c
+++ b/drivers/gpu/drm/nouveau/nv50_display.c
@@ -110,8 +110,8 @@
 		return ret;
 	}
 
-	ret = nouveau_mem_init_heap(&chan->ramin_heap, chan->ramin->gpuobj->
-				    im_pramin->start, 32768);
+	ret = drm_mm_init(&chan->ramin_heap,
+			  chan->ramin->gpuobj->im_pramin->start, 32768);
 	if (ret) {
 		NV_ERROR(dev, "Error initialising EVO PRAMIN heap: %d\n", ret);
 		nv50_evo_channel_del(pchan);