ARM and Imagination used to work together.
ARM used to sell the combination of ARM processors and Imagination graphics into the mobile phones with some success. That's when ARM decided it wanted to have all the cake and went out and bought Falanx Microsystems and turned them into its GPU unit.
Don't see why ARM would want to acquire AMD. AMD does not have anything to offer - no longer has a fab, its new server parts do not compete with the current Intel ones and its design execution is not the best in the industry. ATI graphics? maybe but ARM already has its own GPU offerings.