drm/i915: fix oops on agp=off

From Kernel BZ 10289 - not sure why anyone would boot an intel with no agp
but it shouldn't crash.

Signed-off-by: Dave Airlie <[e-mail]>

drivers/char/drm/i915_dma.c

···
 {
 	drm_i915_private_t *dev_priv = dev->dev_private;
 
+	if (!dev_priv)
+		return;
+
 	if (dev_priv->agp_heap)
 		i915_mem_takedown(&(dev_priv->agp_heap));