radeong: Make sure that we're not emitting relocations for local buffers.

Split from Marek's immd-mode patch.
diff --git a/src/gallium/winsys/drm/radeon/core/radeon_r300.c b/src/gallium/winsys/drm/radeon/core/radeon_r300.c
index 0253bc2..d759bea 100644
--- a/src/gallium/winsys/drm/radeon/core/radeon_r300.c
+++ b/src/gallium/winsys/drm/radeon/core/radeon_r300.c
@@ -81,9 +81,13 @@
                                   uint32_t flags)
 {
     int retval = 0;
+    struct radeon_pipe_buffer* radeon_buffer =
+        (struct radeon_pipe_buffer*)pbuffer;
 
-    retval = radeon_cs_write_reloc(winsys->priv->cs,
-            ((struct radeon_pipe_buffer*)pbuffer)->bo, rd, wd, flags);
+    assert(!radeon_buffer->pb);
+
+    retval = radeon_cs_write_reloc(winsys->priv->cs, radeon_buffer->bo,
+                                   rd, wd, flags);
 
     if (retval) {
         debug_printf("radeon: Relocation of %p (%d, %d, %d) failed!\n",