disp: msm: sde: use prepare_commit connector callback for DSI

Some DSI panels require qsync on/off DCS commands to be sent in
prepare commit phase. Add the hook for sending these commands
to the panel. If these commands are not present in the panel's
device tree node, it will be treated as a NO-OP.

Change-Id: Ida65512fe0214779dfa848f6c7b33644d3934afd
Signed-off-by: Steve Cohen <cohens@codeaurora.org>