board-m7-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 lowest frequency, and
odd behavior from the GPU load measurement algorithm - low load
reported despite awful performance.  At 1500 MBPS bus speed, UI
responsiveness issues at lowest frequency disappear and the GPU will scale
properly when it needs to.

Change-Id: Icd28b1c398e00803ea811a7f0cbb4e89280210d1