drm/amdgpu: switch to aqua_vanjaram_doorbell_index_init

New doorbell index assignment is used by aqua_vanjaram.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>