drm/amdgpu: Initialize asic functions for Arcturus

After cherry-picking doorbell rework changes from drm-next
branch, Arcturus asic functions pointer need to be initialized
to init doorbell index for Arcturus.

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