drm/amd/amdgpu: Fix incorrect logic to increment VCN doorbell index

- The original logic uses a counter based index assignment,
  which is incorrect if we only assign VCN1 to this VF but no VCN0

  The doorbell index is absolute, so we can calculate it by
  using index variable i and j

Signed-off-by: Bokun Zhang <Bokun.Zhang@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>