msm: display: Handle mixer stage commit for wfd correctly

If no dequeue buffer is present, do not return early as commit
call still needs to take care of  mixer staging and stage commit.
Early return of commit, results in pipe still staged errors since,
mixer stage and commit gets bypass.

Change-Id: I1aa7dc9e95d183b22739daae123cf7818afbeadd
CRs-Fixed:523959
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>