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: f122712b42
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>

OvmfPkg/Library/AcpiTimerLib/BaseAcpiTimerLib.c

@@ -63,8 +63,6 @@ AcpiTimerLibConstructor (
       return RETURN_UNSUPPORTED;
   }
 
-  mAcpiTimerIoAddr = (PciRead32 (Pmba) & ~PMBA_RTE) + ACPI_TIMER_OFFSET;
-
   //
   // Check to see if the Power Management Base Address is already enabled
   //
@@ -81,6 +79,7 @@ AcpiTimerLibConstructor (
     PciOr8 (AcpiCtlReg, AcpiEnBit);
   }
 
+  mAcpiTimerIoAddr = (PciRead32 (Pmba) & ~PMBA_RTE) + ACPI_TIMER_OFFSET;
   return RETURN_SUCCESS;
 }