tests/amdgpu: add parentheses to make operation priority explicit

While at it, align with the other half on the next line.

Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>