msm_rotator: add rotator ref count to cleanup code

Perform cleanup of rotator sessions only if reference count reaches 0,
this avoids closing rotator sessions from sibling threads.

Change-Id: I83add90cc04faa90c372dde09a096c7a07be528a
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>