msm_fb: dtv: Serve device off in a separate thread

DTV device should not be powered down until the audio engine has
shutdown. This change moves dtv audio off sequence to a separate
thread so that framebuffer close system call does not get blocked.

CRs-Fixed: 412575
Change-Id: I246a9e58f1367e653913773696510f375abf3d3a
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>