OvmfPkg/IoMmuDxe: IoMmuUnmap(): clean up DEBUG message

OvmfPkg/IoMmuDxe: IoMmuUnmap(): clean up DEBUG message

The only important external information for this function, and for the
human looking at the log, is the Mapping input parameter. Log it on entry.

Stop logging the contents of the MAP_INFO structure pointed-to by Mapping.
Thanks to the previous patch, we can now associate IoMmuUnmap() messages
with IoMmuMap() messages -- and thereby with MAP_INFO contents -- purely
via Mapping.

Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>