hw/display/xlnx_dp: Move problematic code from instance_init to realize

hw/display/xlnx_dp: Move problematic code from instance_init to realize

aux_create_slave() calls qdev_init_nofail() which in turn "realizes"
the corresponding object. This is unlike qdev_create(), and it is wrong
because qdev_init_nofail() must not be called from an instance_init
function.  Move qdev_init_nofail() and the subsequent aux_map_slave into
the caller's realize function.

There are two more bugs that needs to be fixed here, too, where the
objects are created but not added as children.  Therefore when
you call object_unparent on them, nothing happens.

In particular dpcd and edid give you an infinite loop in bus_unparent,
because device_unparent is not called and does not remove them from
the list of devices on the bus.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1531745974-17187-17-git-send-email-thuth@redhat.com
[thuth: Added Paolo's fixup for the dpcd and edid unparenting]
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>