msm: vidc: Consolidate packetization boiler-plate code into a function

Forming the hfi_enable packet is fairly simple and has been copy-pasted
several times into the file.  Consolidate it into a separate function to
reduce SLOC.

Change-Id: I8efeb06de05808d94bf0e3d0e91a36ea74537763
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>