drm/msm: Move call to PTR_ERR_OR_ZERO after reassignment

Here, a location is reset to NULL before being passed to PTR_ERR.
So, PTR_ERR should be called before its argument is reassigned
to NULL. Further to simplify things use PTR_ERR_OR_ZERO instead
of PTR_ERR and IS_ERR.

Problem found using Coccinelle.

Signed-off-by: Vaishali Thakkar <vaishali.thakkar@oracle.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
[fixed fmt string warning (s/%ld/%d/)]
Signed-off-by: Rob Clark <robdclark@gmail.com>