SourceLevelDebugPkg/DebugCommLibUsb3Pei: Make sure alloc physical mem

PI 1.6 has supported pre permanent memory page allocation,
to make sure the allocated memory is physical memory for DMA,
the patch is to check memory discovered PPI installed or not first
before calling AllocatePages.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>