msm: sde: Add mutex lock for debug buffer access in rotator

Adding mutex lock access protection to debug buffer for SDE rotator. The
buffer can be shared between multiple processes, and it is possible that
one process try to free the buffer while another process is still
accessing it.

CRs-Fixed: 2003129
Change-Id: Ib20767f02ba7f14fb972d5c50ab264b2309a1ec2
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>