msm: camera: add BE stats support for vfe40

1.Configure VFE Bayer Exposure(BE) stats registers to collect BE
  stats for 3A to decide total gain to apply on camera.
2.Add buffer support to collect Bayer Exposure stats for 3A to use.
3.Allocate and release buffer properly for BE stats.

Change-Id: Ib857173aa195b9ff1d8b44cf7ba5f9a4a0d30621
Signed-off-by: Peter Liu <pingchie@codeaurora.org>