[libdrm] amdgpu/: concisely && consistently check null ptrs in canonical form

[Emil Velikov]

Hi Edward,

On 18 April 2017 at 17:20, Edward O'Callaghan
<funfunctor at folklore1984.net> wrote:
[snip]
>         if (NULL == sem->signal_fence.context)
>                 return -EINVAL;
I think this hunk can be fixed as well.

-Emi