msm: wfd: Clean up code in mdp subdevice

Just a general cleanup of code in the mdp5 subdevice.  Also fixed a minor
bug in which we called msm_fb_get_writeback_fb() twice for no good
reason.

Change-Id: I805f5f2e5e04d365cd0d93281a0c835940cea0fe
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>