Register display plugin dbus handlers earlier

Introspection data is available after dbus handlers are registered.
Sending signals before handlers are in place causes warnings about
non-introspectable signals to be emitted to the journal.

Move handler registration to occur earlier in display plugin init.

Also move initial display brightness probing into a separate function.

[mce] Register display plugin dbus handlers earlier. Fixes JB#22091
Merge pull request #208 from spiiroin/fix_display_init

Register display plugin dbus handlers earlier