UefiCpuPkg/MtrrLib: Fix bug that may incorrectly set <1MB attribute

UefiCpuPkg/MtrrLib: Fix bug that may incorrectly set <1MB attribute

MtrrLibSetBelow1MBMemoryAttribute() may be called multiple times.
It's possible that in a 2nd call, Modified[0] is set to TRUE in
1st call but ClearMasks[0] and OrMasks[0] is uninitialized in
2nd call. It causes FixedSettings->Mtrr[0] be set to random
data.

The patch fixes this issue by introducing a local Modified[]
array and only updates FixedSettings->Mtrr[] when LocalModified[i]
is TRUE.

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