drm: sti: Fix return value check in sti_drm_platform_probe()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>