video: fbdev: imxfb: fix return value check in imxfb_probe()

If devm_ioremap_resource() fails, it never return NULL, replace
NULL test with IS_ERR().

Fixes: b083c22d ("video: fbdev: imxfb: Convert request_mem_region + ioremap to devm_ioremap_resource")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>