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

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

Fixes: 8223286d ("drm/msm: Add a helper function for in-kernel
buffer allocations")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>