Change Nexus 5 and Nexus 6 deep buffer to 2 periods of 1920 frames(40ms)

Prior setting of 8 periods of 960 frames had identical
thread processing interval to the normal mixer thread.
By increasing the thread processing interval, there is
a measured power reduction of 10 percent on MSM8994.

Note: as a consequence, operations such as pause or volume
may have a slightly greater granularity (40 ms vs 20 ms).

Bug: 21198655
Change-Id: Icd8ce12c987d2b89578c2d2ad9a2ded54c6ce4c0