OvmfPkg/AcpiTimerLib: don't use possibly unset PMBA register (PEI phase)

OvmfPkg/AcpiTimerLib: don't use possibly unset PMBA register (PEI phase)

We should store the right value to the PMBA (if the PMBA needs
initialization) before setting mAcpiTimerIoAddr from the PMBA.

Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Fixes: f122712b42af3d63902c2c141e90d2c200df6790
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>