OMAPDSS: DPI: fix crash with dpi_verify_dsi_pll()

If the DSI support has not been compiled in or the SoC doesn't have DSI
hardware, dpi_get_dsidev() returns NULL. This NULL is passed to
dpi_verify_dsi_pll() causing a crash. The bug was added with commit
0e8276ef75f5c7811b038d1d23b2b42c16efc5ac (OMAPDSS: DPI: always use DSI
PLL if available).

Fix this by checking if dsidev is NULL before calling
dpi_verify_dsi_pll().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/dss/dpi.c

@@ -412,7 +412,7 @@ static int __init dpi_init_display(struct omap_dss_device *dssdev)
 	 */
 	dsidev = dpi_get_dsidev(dssdev->channel);
 
-	if (dpi_verify_dsi_pll(dsidev)) {
+	if (dsidev && dpi_verify_dsi_pll(dsidev)) {
 		dsidev = NULL;
 		DSSWARN("DSI PLL not operational\n");
 	}