video: ARM CLCD: fix return value check in versatile_clcd_init_panel()

In case of error, the function syscon_node_to_regmap() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Fixes: 25348160 ("video: ARM CLCD: add special panel hook for Versatiles")
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>