monitor-manager-kms: Cope with non-existent connectors and encoders

DRM objects like connectors and encoders might change at any time, in
particular they might become invalid between drmModeGetResources() and
getting the actual objects in which case they'll be NULL. Be defensive
against that.

Note that, if this happens, we should get another udev event soon
which will cause us to update our state.

https://bugzilla.gnome.org/show_bug.cgi?id=745476