UefiCpuPkg/CpuMpPei: Correct BIST PPI logic.

UefiCpuPkg/CpuMpPei: Correct BIST PPI logic.

Currently, the SecPlatformInformation2 PPI is installed when either
there is none present or the present one doesn't lack data.
Update the logic to only install the SecPlatformInformation2 PPI when
it's not already installed so that an up-to-date PPI remains the only
one and unchanged.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>