dm: core: Replace of_offset with accessor

At present devices use a simple integer offset to record the device tree
node associated with the device. In preparation for supporting a live
device tree, which uses a node pointer instead, refactor existing code to
access this field through an inline function.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/video/tegra124/display.c b/drivers/video/tegra124/display.c
index 28db96c..bbbca13b 100644
--- a/drivers/video/tegra124/display.c
+++ b/drivers/video/tegra124/display.c
@@ -363,9 +363,9 @@
 		return ret;
 	}
 
-	dc_ctlr = (struct dc_ctlr *)fdtdec_get_addr(blob, dev->of_offset,
+	dc_ctlr = (struct dc_ctlr *)fdtdec_get_addr(blob, dev_of_offset(dev),
 						    "reg");
-	if (fdtdec_decode_display_timing(blob, dev->of_offset, 0, timing)) {
+	if (fdtdec_decode_display_timing(blob, dev_of_offset(dev), 0, timing)) {
 		debug("%s: Failed to decode display timing\n", __func__);
 		return -EINVAL;
 	}