msm: mdss: control both dsi controller's clock atomically

For split display case there has possibilities of race condition
may happen during dsi clocks control between add_vsync and
clokc_off thread which could end dsi clock is turned off
instead of turn on as add_vsycn thread expect. Therefore
clocks control of both dsi controllers should be atomic.

CRs-Fixed: 724861
Change-Id: I537502bad2611769d5323cd05ed50c505af6371a
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>