msm: ipc: Refactor the code that processes control messages

process_control_msg function looks lengthy and heavily indented.

Refactor the code that processes each type of control message into a
separate function of its own.

Change-Id: Id7c4038dd4369a182ed855bd6b1e66ce07f5b8c3
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>