drm/msm: fix return value check in ERR_PTR()

In case of error, the function drm_prime_pages_to_sg() 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>