UefiCpuPkg/MtrrLib: Fix a MTRR calculation bug

UefiCpuPkg/MtrrLib: Fix a MTRR calculation bug

80                   A8   B0   B8   C0
+----------WB--------+-UC-+-WT-+-WB-+

For above memory settings, current code caused the final MTRR
settings miss [A8, B0, UC] when default memory type is UC.

The root cause is the code only checks the mandatory weight
between A8 to B0, but skips to check the optional weight.
The patch fixes this issue.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>