Enable triple framebuffers

On high resolution panels, GPU composition can end up taking more
time than the vsync period, thereby causing jank. Enable triple
framebuffers by default on 8960 to allow GPU one extra frame to
work with. No latency increase is observed with this change in
regular use cases.

Change-Id: Ie1f9b5b34c6058f3358e19c3e494a67b89a8c0e7