power: Add support for SET_PROFILE hints

 * Add support for system performance profiles.
 * Uses mpctl to configure the CPUs in the appropriate modes.
 * PERFORMANCE = all cores at max
 * POWER_SAVE = max two cores at non-turbo voltage
 * If a custom profile is set, don't honor any other hints
 * Clean up the code and firm up the locking.

Change-Id: Ie6acada805780c9ae6e6bc2002843aef638ca63b
power: Disable the 8974pro-specific display on/off path

 * It's now handled by mpctl directly, and this code does not correctly
   handle the interactive governor. Just disable it as the main
   function does everything we need.

Change-Id: I8fdf9d80bfbf940b55bbe962c1e7a0829bcec14c