drm/amdgpu: Revert "add mutex for ba_va->valids/invalids"

Not needed any more because we need to protect the elements on the list anyway.

This reverts commit 38bf516c75b4ef0f5c716e05fa9baab7c52d6c39.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>