drm/panel: himax-hx8394: Handle errors from mipi_dsi_dcs_set_display_on() better

If mipi_dsi_dcs_set_display_on() returned an error then we'd store
that in the "ret" variable and jump to error handling. We'd then
attempt an orderly poweroff. Unfortunately we then blew away the value
stored in "ret". That means that if the orderly poweroff actually
worked then we're return 0 (no error) from hx8394_enable() even though
the panel wasn't enabled.

Fix this by not blowing away "ret".

Found by code inspection.

Fixes: 65dc9360 ("drm: panel: Add Himax HX8394 panel controller driver")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240517143643.1.I0a6836fffd8d7620f353becb3df2370d2898f803@changeidSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517143643.1.I0a6836fffd8d7620f353becb3df2370d2898f803@changeid

drivers/gpu/drm/panel/panel-himax-hx8394.c

@@ -370,8 +370,7 @@ static int hx8394_enable(struct drm_panel *panel)
 
 sleep_in:
 	/* This will probably fail, but let's try orderly power off anyway. */
-	ret = mipi_dsi_dcs_enter_sleep_mode(dsi);
-	if (!ret)
+	if (!mipi_dsi_dcs_enter_sleep_mode(dsi))
 		msleep(50);
 
 	return ret;