msm: wfd: Fix bug in calcuating number of MDP updates

The mdp_updates variable is getting incremented when giving a buffer to
MDP subdev.  It should be getting incremented when we get it back from
the subdev.

Change-Id: Ib2c6ea45ef63c85b1bed26cd4d4948d4ee1a927a
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>