drm/amdgpu: remove the redundant null check

[Markus Elfring]

>> Were any source code analysis tools involved for finding
>> these update candidates?
> With the help of Coccinelle. You can find out some example in scripts/coccinelle/.

Thanks for such background information.
Was the script “ifnullfree.cocci” applied here?

Will it be helpful to add attribution for such tools
to any more descriptions in your patches?

Regards,
Markus