board-8064-gpu: Increase bus freq at lowest GPU frequency

1000 MBPS is too low for a 1920x1080 panel.  This results in
major UI responsiveness issues when the GPU is at 128 MHz, and
odd behavior from the GPU load measurement algorithm - low load
reported despite awful performance.  At 1500 MBPS bus speed, UI
responsiveness issues at 128 MHz disappear and the GPU will scale
properly when it needs to.

Change-Id: Iea8008d1b106ec4dd172d479d68b99f5d10b3317