Vlv2TbltDevicePkg/PpmPolicy: Remove the usage of global variables

Vlv2TbltDevicePkg/PpmPolicy: Remove the usage of global variables

gcc issued the error of "multiple deifintion" since gBS was also defined
in MdePkg/Library/UefiBootServicesTableLib. Actually those global variables,
gBS, pBS, and pRS, in PpmPolicy.h were only used in one function. Besides,
gBS and pRS were not really used. Remove gBS and pRS and declare pBS in
PpmPolicyEntry() to satisfy gcc.

Cc: David Wei <david.wei@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: David Wei <david.wei@intel.com>