msm: mdss: stop display commit thread only if it is running

When the framebuffer device is shutdown, it is possible that
the display commit thread has already been stopped. In such
scenarios, ensure that we not attempt to stop the thread again
to avoid any potential crashes.

CRs-Fixed: 584499
Change-Id: If0026cdcc4367c149815933ffe4ed08056d54770
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>