nvidia/drivers: dc: fix return code in tegra_dc_wait_for_vsync

The return code from wait_for_completion_interruptible_timeout()
was being returned directly as an error code (to userspace through
the FBIO_WAITFORVSYNC ioctl), instead of being translated, so
change the logic:

 * loop while the return code is -ERESTARTSYS due to interruption
 * 0 return is timeout, so return -ETIME
 * > 0 is success

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: CTCaer <ctcaer@gmail.com>

drivers/video/tegra/dc/dc.c

@@ -4493,14 +4493,18 @@ int tegra_dc_wait_for_vsync(struct tegra_dc *dc)
 	timeout_ms = DIV_ROUND_UP(2 * 1000000, refresh);
 	_tegra_dc_user_vsync_enable(dc, true);
 	mutex_unlock(&dc->lock);
-	ret = wait_for_completion_interruptible_timeout(
-		&dc->out->user_vblank_comp, msecs_to_jiffies(timeout_ms));
+	do {
+		ret = wait_for_completion_interruptible_timeout(
+			&dc->out->user_vblank_comp, msecs_to_jiffies(timeout_ms));
+	} while (ret == -ERESTARTSYS);
 	mutex_lock(&dc->lock);
+	if (ret == 0)
+		ret = -ETIMEDOUT;
 	_tegra_dc_user_vsync_enable(dc, false);
 out:
 	mutex_unlock(&dc->lock);
 	mutex_unlock(&dc->lp_lock);
-	return ret;
+	return (ret >= 0 ? 0 : ret);
 }
 
 int _tegra_dc_wait_for_frame_end(struct tegra_dc *dc,