I do it two ways, depending on where I am.
For the icons on your desktop, hold down Control and Option together while clicking on one of the partition icons. The command list will include an option to eject that partition. If you keep holding Option but release Control, that option will change to eject all of the partitions (on that drive). It's easier than it sounds.
In Finder, even easier still. In the sidebar, just click one of the Eject icons while holding the Option key. All of the partitions on that drive will unmount.