drm/panel: st7701: Add prepare_prev_first flag to drm_panel

The DSI host must be enabled for the panel to be initialized in
prepare(). Set the prepare_prev_first flag to guarantee this.
This fixes the panel operation on NXP i.MX8MP SoC / Samsung DSIM
DSI host.

Fixes: 849b2e3ff969 ("drm/panel: Add Sitronix ST7701 panel driver")
Signed-off-by: Marek Vasut <[e-mail]>
Reviewed-by: Jessica Zhang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panel/panel-sitronix-st7701.c

···
 		return dev_err_probe(dev, ret, "Failed to get orientation\n");
 
 	drm_panel_init(&st7701->panel, dev, &st7701_funcs, connector_type);
+	st7701->panel.prepare_prev_first = true;
 
 	/**
 	 * Once sleep out has been issued, ST7701 IC required to wait 120ms