msm: mdp: Change msmfb kworker to kthread

This worker can be a longer running task depending on the number
of frames queued, hence change it to a thread.

Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>
Change-Id: Iebf7e95d0486ac43aa6a12d70442936600c0249a

Conflicts:

	drivers/video/msm/msm_fb.c