disp: msm: sde: setting async cmd wait flag only for DSI

Asynchronous command transfer wait during pre kickoff
is only applicable for DSI. The change ensures that
the flag is set only for DSI connector, otherwise it can
result in memory scribbling for other connectors.

Change-Id: I623f15cf13fcd3ae72f584d5ef8883570a848c93
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>