msm_fb: display: initialize dmap_comp before wait4dmap

blt_wait flag will be set when both two writeback buffers
are used. In this case, kickoff can not be proceeded until
next dmap_done to release writeback buffer. This patch
initialize dmap_comp before wait4dmap to make sure
wait_for_completion is enforced.

CRs-fixed: 448168
Change-Id: I64f2ad599b18f9d8b346c41a55829c564fcced0b
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>