drm/amdgpu: Add asic func interface to init doorbell index

This is preparation to move doorbell index initialization
to amdgpu_asic_funcs

Signed-off-by: Oak Zeng <ozeng@amd.com>
Proposed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>