drm: use NULL instead of 0

From: Randy Dunlap <rdunlap@xenotime.net>

Use NULL instead of 0 (sparse warnings):

drivers/char/drm/ati_pcigart.c:64:10: warning: Using plain integer as NULL
pointer
drivers/char/drm/ati_pcigart.c:130:21: warning: Using plain integer as NULL
pointer
drivers/char/drm/ati_pcigart.c:171:14: warning: Using plain integer as NULL
pointer
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/ati_pcigart.c

@@ -61,7 +61,7 @@ static void *drm_ati_alloc_pcigart_table(void)
 
 	address = __get_free_pages(GFP_KERNEL, ATI_PCIGART_TABLE_ORDER);
 	if (address == 0UL) {
-		return 0;
+		return NULL;
 	}
 
 	page = virt_to_page(address);
@@ -127,7 +127,7 @@ int drm_ati_pcigart_cleanup(drm_device_t *dev, drm_ati_pcigart_info *gart_info)
 	if (gart_info->gart_table_location == DRM_ATI_GART_MAIN
 	    && gart_info->addr) {
 		drm_ati_free_pcigart_table(gart_info->addr);
-		gart_info->addr = 0;
+		gart_info->addr = NULL;
 	}
 
 	return 1;
@@ -168,7 +168,7 @@ int drm_ati_pcigart_init(drm_device_t *dev, drm_ati_pcigart_info *gart_info)
 		if (bus_address == 0) {
 			DRM_ERROR("unable to map PCIGART pages!\n");
 			drm_ati_free_pcigart_table(address);
-			address = 0;
+			address = NULL;
 			goto done;
 		}
 	} else {