drums in piano roll view

Go to your MIDI track view.
Highlight the pattern you wish to use, then right click, and select Enable Groove Clip Looping. It's a good idea to have Snap to Grid set to at least Quarter notes or bigger, to keep it from drifting out of time.
At this point, your clip should have all four corners clipped, indicating a loop. Grab the right edge, and drag it out as far as you want.
You should get a consistent, repeating pattern this way.