ply-boot-splash: Do not add ply_boot_splash_update_progress timeout multiple times

ply-boot-splash: Do not add ply_boot_splash_update_progress timeout multiple times

Before this commit when freeing the splash, the following would be logged:

multiple matching timeouts found for removal
multiple matching timeouts found for removal

This is caused by us adding the ply_boot_splash_update_progress timeout
handler to the event loop 3 times: 1 on first show, 2 on second show with
a different mode, 3 on becoming idle.

This commit fixes the 2nd add by stopping the timer when changing modes
and the 3th add by not calling ply_boot_splash_update_progress to update
the progress, as that will re-add itself. Instead this commit directly calls
plugin_interface->on_boot_progress from ply_boot_splash_become_idle.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>