msm: mdss: refactor ping pong completion logic

Replace completion structures with waitq and track the completion of
ping pong transfer with a single kickoff count variable. This prevents
race conditions when using completion structure and provides a cleaner
way to identify whether wait is required.

CRs-Fixed: 720541
Change-Id: Iebb8077d520649db427470fc4963d892967a920c
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>