[AGPGART] Fix sparse NULL pointer warnings.

Signed-off-by: Dave Jones <davej@redhat.com>

drivers/char/agp/generic.c

@@ -917,7 +917,7 @@ void *agp_generic_alloc_page(void)
 
 	page = alloc_page(GFP_KERNEL);
 	if (page == NULL)
-		return 0;
+		return NULL;
 
 	map_page_into_agp(page);
 

drivers/char/agp/intel-agp.c

@@ -369,7 +369,7 @@ static int intel_i830_create_gatt_table(void)
 	size = agp_bridge->current_size;
 	page_order = size->page_order;
 	num_entries = size->num_entries;
-	agp_bridge->gatt_table_real = 0;
+	agp_bridge->gatt_table_real = NULL;
 
 	pci_read_config_dword(intel_i830_private.i830_dev,I810_MMADDR,&temp);
 	temp &= 0xfff80000;

drivers/char/agp/intel-mch-agp.c

@@ -167,7 +167,7 @@ static int intel_i830_create_gatt_table(void)
 	size = agp_bridge->current_size;
 	page_order = size->page_order;
 	num_entries = size->num_entries;
-	agp_bridge->gatt_table_real = 0;
+	agp_bridge->gatt_table_real = NULL;
 
 	pci_read_config_dword(intel_i830_private.i830_dev,I810_MMADDR,&temp);
 	temp &= 0xfff80000;