msm: camera: isp: fix two corner case handling for the framedrop pattern

This change fixes two corner case handling for the framedrop pattern

a) return the buffer when it is required in the drop frame handling

b) correct an incorrect framedrop pattern update when we miss the reg
   update

Change-Id: I6d9b7715d3f0545a673d70e7744141f92bbc3a9d
Signed-off-by: Jing Zhou <jzhou70@codeaurora.org>