drm: fix all sparse warning on 32-bit x86

Finally cleaned up the sparse warnings for the drm.

Signed-off-by: Dave Airlie <airlied@linux.ie>
diff --git a/drivers/char/drm/drm_context.c b/drivers/char/drm/drm_context.c
index 6380127..bdd168d 100644
--- a/drivers/char/drm/drm_context.c
+++ b/drivers/char/drm/drm_context.c
@@ -232,7 +232,7 @@
 	map = dev->context_sareas[request.ctx_id];
 	up(&dev->struct_sem);
 
-	request.handle = 0;
+	request.handle = NULL;
 	list_for_each_entry(_entry, &dev->maplist->head, head) {
 		if (_entry->map == map) {
 			request.handle =
@@ -240,7 +240,7 @@
 			break;
 		}
 	}
-	if (request.handle == 0)
+	if (request.handle == NULL)
 		return -EINVAL;
 
 	if (copy_to_user(argp, &request, sizeof(request)))