MdeModulePkg/Core/Dxe/Gcd: make persistent override special-purpose

Fix the GCD memory type selection logic in DXE GCD initialization so persistent memory correctly takes precedence over special-purpose memory when both attributes are present.

The existing code/comment said persistent should win, but the condition order allowed special-purpose to overwrite persistent. This change swaps the checks so behavior matches the intended precedence and comment.

Signed-off-by: Sureshkumar Ponnusamy <sponnusamy@microsoft.com>