UefiCpuPkg/MtrrLib: Update MtrrGetFixedMtrr().

IsMtrrSupported() return true when either fixed mtrr supported or
variable mtrr suppored. In this case, rather than checking whether
MTRR is supported or not, we should specifically check for fixed MTRR
support.

Signed-off-by: Yuanhao Xie <yuanhao.xie@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Ray Ni <ray.ni@intel.com>

Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>

UefiCpuPkg/Library/MtrrLib/MtrrLib.c

@@ -468,7 +468,11 @@ MtrrGetFixedMtrr (
   OUT MTRR_FIXED_SETTINGS  *FixedSettings
   )
 {
-  if (!IsMtrrSupported ()) {
+  BOOLEAN  FixedMtrrSupported;
+
+  MtrrLibIsMtrrSupported (&FixedMtrrSupported, NULL);
+
+  if (!FixedMtrrSupported) {
     return FixedSettings;
   }