drm/amdgpu: initialize gfxhub v1_2 and mmhub v1_8 funcs

Initialize gfxhub1.2 and mmhub1.8 function calls

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>