dm: i2c: Rename driver model I2C functions to permit compatibility

Add a dm_ prefix to driver model I2C functions so that we can keep the old
ones around.

This is a little unfortunate, but on reflection it is too difficult to
change the API. We can undo this rename when most boards and drivers are
converted to use driver model for I2C.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/nvidia/whistler/whistler.c b/board/nvidia/whistler/whistler.c
index 3114b20..ad6ea09 100644
--- a/board/nvidia/whistler/whistler.c
+++ b/board/nvidia/whistler/whistler.c
@@ -33,15 +33,15 @@
 		return;
 	}
 	val = 0x29;
-	ret = i2c_write(dev, 0x46, &val, 1);
+	ret = dm_i2c_write(dev, 0x46, &val, 1);
 	if (ret)
 		printf("i2c_write 0 0x3c 0x46 failed: %d\n", ret);
 	val = 0x00;
-	ret = i2c_write(dev, 0x45, &val, 1);
+	ret = dm_i2c_write(dev, 0x45, &val, 1);
 	if (ret)
 		printf("i2c_write 0 0x3c 0x45 failed: %d\n", ret);
 	val = 0x1f;
-	ret = i2c_write(dev, 0x44, &val, 1);
+	ret = dm_i2c_write(dev, 0x44, &val, 1);
 	if (ret)
 		printf("i2c_write 0 0x3c 0x44 failed: %d\n", ret);
 
@@ -70,11 +70,11 @@
 		return;
 	}
 	val = 0x03;
-	ret = i2c_write(dev, 2, &val, 1);
+	ret = dm_i2c_write(dev, 2, &val, 1);
 	if (ret)
 		printf("i2c_write 0 0x20 2 failed: %d\n", ret);
 	val = 0xfc;
-	ret = i2c_write(dev, 6, &val, 1);
+	ret = dm_i2c_write(dev, 6, &val, 1);
 	if (ret)
 		printf("i2c_write 0 0x20 6 failed: %d\n", ret);
 }