drm/msm/sde: initialize flush mask for command encoder

Flush mask is not initialized and sets random flush
bits during command encoder flush request. Initialize flush
mask to avoid incorrect flush request.

CRs-Fixed: 2009714
Change-Id: I76723f45d8fc92c9b7fa9ac496a4b34938204484
Signed-off-by: Alan Kwong <akwong@codeaurora.org>