msm_fb: display: use spin_lock_bh() when turns on/off dsi_clk

Since dsi clock is accessed by both thread and timer context,
spin_lcok_bh()/spin_unlcok_bh() must be used to serialize accessing
of dsi clock.

Change-Id: Iac80400d871c90413dcbb10a12efc6bc6247e986
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>