OvmfPkg/QemuFlashFvbServicesRuntimeDxe: use Mmio helper to clear enc mask

OvmfPkg/QemuFlashFvbServicesRuntimeDxe: use Mmio helper to clear enc mask

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

Use the MemEncryptSevClearMmioPageEncMask() to clear memory encryption mask
for the Mmio address range.

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: Laszlo Ersek <lersek@redhat.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Message-Id: <20210519181949.6574-12-brijesh.singh@amd.com>