intel-agp: fix a panic with 1M of shared memory, no GTT entries

When GTT size is equal to amount of video memory, the amount of GTT
entries is computed lower than zero, which is invalid and leads to
off-by-one error in intel_i915_configure()

Originally posted here:
http://bugzilla.kernel.org/show_bug.cgi?id=12539
http://bugzilla.redhat.com/show_bug.cgi?id=445592Signed-off-by: NLubomir Rintel <lkundrak@v3.sk>
Cc: Lubomir Rintel <lkundrak@v3.sk>
Cc: Dave Airlie <airlied@linux.ie>
Reviewed-by: NEric Anholt <eric@anholt.net>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/agp/intel-agp.c

@@ -633,13 +633,15 @@ static void intel_i830_init_gtt_entries(void)
 			break;
 		}
 	}
-	if (gtt_entries > 0)
+	if (gtt_entries > 0) {
 		dev_info(&agp_bridge->dev->dev, "detected %dK %s memory\n",
 		       gtt_entries / KB(1), local ? "local" : "stolen");
-	else
+		gtt_entries /= KB(4);
+	} else {
 		dev_info(&agp_bridge->dev->dev,
 		       "no pre-allocated video memory detected\n");
-	gtt_entries /= KB(4);
+		gtt_entries = 0;
+	}
 
 	intel_private.gtt_entries = gtt_entries;
 }