msm: sde: avoid dynamic kthread create for each rot session

Each rotator session open request creates a thread and
destroys it on close session. These threads are created
to trigger rotation complete fences. Such dynamic allocation
can easily be avoided by creating threads at probe time.

Change-Id: I1a443ca8627b5cf2b2e770ad7ea2863c52aa7d9b
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>