drm/amdgpu: initialize cg/pg flags and external rev id for navi12

don't enable any cg/pg features yet.

v2: calculate external revision id from revision id so that we can
    differentiate navi12 A0 from A1 directly.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>