video: tegra20: dsi: use regulator_set_enable_if_allowed

With the commit 4fcba5d556b4 ("regulator: implement basic reference
counter") the return value of regulator_set_enable may be EALREADY or
EBUSY for fixed/gpio regulators and may be further expanded on all
regulators.

Change to use the more relaxed regulator_set_enable_if_allowed to
continue if regulator already was enabled or disabled.

Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
diff --git a/drivers/video/tegra20/tegra-dsi.c b/drivers/video/tegra20/tegra-dsi.c
index 8c3404e..b4cf4fa 100644
--- a/drivers/video/tegra20/tegra-dsi.c
+++ b/drivers/video/tegra20/tegra-dsi.c
@@ -831,11 +831,9 @@
 
 	tegra_dsi_get_format(device->format, &priv->format);
 
-	if (priv->avdd) {
-		ret = regulator_set_enable(priv->avdd, true);
-		if (ret)
-			return ret;
-	}
+	ret = regulator_set_enable_if_allowed(priv->avdd, true);
+	if (ret && ret != -ENOSYS)
+		return ret;
 
 	tegra_dsi_init_clocks(dev);