xen kconfig: relax INPUT_XEN_KBDDEV_FRONTEND deps

PV-on-HVM guests may want to use the xen keyboard/mouse frontend, but
they don't use the xen frame buffer frontend. For this case it doesn't
make much sense for INPUT_XEN_KBDDEV_FRONTEND to depend on
XEN_FBDEV_FRONTEND. The opposite direction always makes more sense, i.e.
if you're using xenfb, then you'll want xenkbd. Switch the dependencies.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>