drm/vmwgfx: Use the backdoor port if the HB port is not available

commit cc0ba0d8624f210995924bb57a8b181ce8976606 upstream.

The HB port may not be available for various reasons. Either it has been
disabled by a config option or by the hypervisor for other reasons.
In that case, make sure we have a backup plan and use the backdoor port
instead with a performance penalty.

Cc: stable@vger.kernel.org
Fixes: 89da76fd ("drm/vmwgfx: Add VMWare host messaging capability")
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>