msm: mdss: Add lock to avoid release of active session in rotator

Add mutex lock to protect an active rotator session from being closed.
Without the lock, this can happen if a rotator closing IOCTL is
called from a separate thread.

CRs-Fixed: 2006159
Change-Id: I927a0c626bdae5ef149e12979ec4befdbac1b7f7
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>