msm_fb: display: add mutex to dsi_on

During resume, backlight dcs commands to client may be issued
before dsi_on completed. Add mutex to dsi_on and check dsi state
to make sure dsi_on is completed before sending backlight command
to panel to avoid conflict on dsi link. Otherwise screen will
stay blank if confliction happened.

Change-Id: Icfba079b228f02aeb89949befb10cca29bf6d744
Signed-off-by: kuogee hsieh <khsieh@codeaurora.org>