ui: rbd: storage: obsolete strange getBinds binding assembly

ui: rbd: storage: obsolete strange getBinds binding assembly

this was added for our (also in other places used):
> xtype: me.isCreate ? 'pveCephPoolSelector' : 'displayfield',
construct. But as displayfield does not has a submitValue setter we
cannot bind it generally.

So add such a setter with a small override and declare the bindings
direct when declaring the items, less side effects and all the
configurations for an item stays in the same place.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>