OvmfPkg/AmdSev: update the fdf to use new workarea PCD

OvmfPkg/AmdSev: update the fdf to use new workarea PCD

The commit 80e67af9afca added support for the generic work area concept
used mainly by the encrypted VMs but missed update the AmdSev package.

Fixes: 80e67af9afca ("OvmfPkg: introduce a common work area")
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Reported-by: Dov Murik <dovmurik@linux.ibm.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dov Murik <dovmurik@linux.ibm.com>
Tested-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>