drm/amdgpu: Vega10 doorbell index initialization

v2: Use enum definition instead of hardcoded value
v3: Remove unused enum definition

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