UefiCpuPkg/MtrrLib: Fixed bug if length is less than Fixed-MTRR range

UefiCpuPkg/MtrrLib: Fixed bug if length is less than Fixed-MTRR range

Currently, if the memory length to be programmed is less than the remaining size
of one Fixed-MTRR supported, RETURN_UNSUPPORTED returned. This is not correct.
This is one regression at 07e889209034ba94bfac9e765b8a50ef344daef2 when we
updated ProgramFixedMtrr() to remove the loop of calculating Fixed-MTRR Mask.

This fix will calculate Right offset in Fixed-MTRR beside left offset. It
supports small length (less than remaining size supported by Fixed-MTRR) to be
programmed.

Cc: Eric Dong <eric.dong@intel.com>
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: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>