ShellPkg/UefiDpLib: Fix a DP cumulative data issue

ShellPkg/UefiDpLib: Fix a DP cumulative data issue

The value of PERF_CUM_DATA.Count and PERF_CUM_DATA.Duration field
keep cumulating on every execution of dp.
Initialize the CumData at dp's entry point.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Cinnamon Shia <cinnamon.shia@hpe.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18727 6f19259b-4bc3-4df7-8a09-765794883524