UefiCpuPkg/MtrrLib: Make use of worker functions to get MTRRs count

UefiCpuPkg/MtrrLib: Make use of worker functions to get MTRRs count

Try to make use of worker functions to get MTRRs count. It could avoid invoking
IsMtrrSupported() for many times.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19155 6f19259b-4bc3-4df7-8a09-765794883524