drm/vmwgfx: Don't use the HB port if memory encryption is active

drm/vmwgfx: Don't use the HB port if memory encryption is active

With memory encryption active, the hypervisor typically can't read the
guest memory using the HB port, since it is encrypted using a key known
only to the guest. In that case fall back to processing 4 bytes at a time
using the ordinary backdoor port.
The other option would be to use unencrypted bounce buffers for the
hypervisor to read out from or write into, but given the limited message
sizes it appears more efficient to just fall back to the ordinary backdoor
port.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>