intel_agp: Don't oops with zero stolen memory

When "onboard video memory" is set do "disabled" in BIOS on Asus P4P800-VM
board (i865G), kernel oopses with memory corruption:
https://bugs.freedesktop.org/show_bug.cgi?id=28430

Fix that by cleanly aborting the initialization.
Signed-off-by: NOndrej Zary <linux@rainbow-software.org>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/char/agp/intel-gtt.c

@@ -797,6 +797,10 @@ static int intel_i830_create_gatt_table(struct agp_bridge_data *bridge)
 
 	/* we have to call this as early as possible after the MMIO base address is known */
 	intel_i830_init_gtt_entries();
+	if (intel_private.gtt_entries == 0) {
+		iounmap(intel_private.registers);
+		return -ENOMEM;
+	}
 
 	agp_bridge->gatt_table = NULL;
 
@@ -1279,6 +1283,11 @@ static int intel_i915_create_gatt_table(struct agp_bridge_data *bridge)
 
 	/* we have to call this as early as possible after the MMIO base address is known */
 	intel_i830_init_gtt_entries();
+	if (intel_private.gtt_entries == 0) {
+		iounmap(intel_private.gtt);
+		iounmap(intel_private.registers);
+		return -ENOMEM;
+	}
 
 	agp_bridge->gatt_table = NULL;
 
@@ -1387,6 +1396,11 @@ static int intel_i965_create_gatt_table(struct agp_bridge_data *bridge)
 
 	/* we have to call this as early as possible after the MMIO base address is known */
 	intel_i830_init_gtt_entries();
+	if (intel_private.gtt_entries == 0) {
+		iounmap(intel_private.gtt);
+		iounmap(intel_private.registers);
+		return -ENOMEM;
+	}
 
 	agp_bridge->gatt_table = NULL;