amdgpu: add amdgpu_bo_va_op_raw

This variant allows the caller full control over flags and size, and
allows passing a NULL bo (for PRT support).

Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: Jerry Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>