drm/panel: ilitek-ili9882t: Check for errors on the NOP in prepare()

The mipi_dsi_dcs_nop() function returns an error but we weren't
checking it in ili9882t_prepare(). Add a check. This is highly
unlikely to matter in practice. If the NOP failed then likely later
MIPI commands would fail too.

Found by code inspection.

Fixes: e2450d32e5fb ("drm/panel: ili9882t: Break out as separate driver")
Signed-off-by: Douglas Anderson <[e-mail]>
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/20240517143643.5.I323476ba9fa8cc7a5adee4c1ec95202785cc5686@changeid
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20240517143643.5.I323476ba9fa8cc7a5adee4c1ec95202785cc5686@changeid

drivers/gpu/drm/panel/panel-ilitek-ili9882t.c

···
 	usleep_range(10000, 11000);
 
 	// MIPI needs to keep the LP11 state before the lcm_reset pin is pulled high
-	mipi_dsi_dcs_nop(ili->dsi);
+	ret = mipi_dsi_dcs_nop(ili->dsi);
+	if (ret < 0) {
+		dev_err(&ili->dsi->dev, "Failed to send NOP: %d\n", ret);
+		goto poweroff;
+	}
 	usleep_range(1000, 2000);
 
 	gpiod_set_value(ili->enable_gpio, 1);