drm/amdgpu: add gfx firmware header v2_0

We need define new firmware header to support
CP RS64 fw.

Signed-off-by: Wenhui Sheng <Wenhui.Sheng@amd.com>
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>