compositor-drm, evdev: don't crash on missing keyboard

Weston's notify_keyboard_focus_*() assume that a keyboard is present, if
they are called. With evdev, there might not always be a keyboard.

Also clean up the variable definition in evdev_notify_keyborad_focus().
I read that function through many times and finally had to grep where
does 'all_keys' come from.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>