drm: exynos: dsi: Mark PHY as optional

The same Samsung MIPI DSIM master can also be used in NXP's
i.MX8M Mini/Nano/Plus SoC.

In i.MX8M Mini/Nano/Plus SoC the DSI Phy requires a MIPI DPHY
bit to reset in order to activate the PHY and that can be done
via upstream i.MX8M blk-ctrl driver.

So, mark the phy get as optional.
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_dsi.c

@@ -1732,7 +1732,7 @@ static int exynos_dsi_probe(struct platform_device *pdev)
 	if (IS_ERR(dsi->reg_base))
 		return PTR_ERR(dsi->reg_base);
 
-	dsi->phy = devm_phy_get(dev, "dsim");
+	dsi->phy = devm_phy_optional_get(dev, "dsim");
 	if (IS_ERR(dsi->phy)) {
 		dev_info(dev, "failed to get dsim phy\n");
 		return PTR_ERR(dsi->phy);