msm: camera: memmgr: Remove the mutex lock for kref variable

kref operation is atmoic operation, so no need to use mutex
to protect it, and the cam_mem_put_cpu_buf is also called
in spinlock context, so we can't use mutex lock in this
function. This change removes the mutex lock for kref
variable protextion.

CRs-Fixed: 3786887
Change-Id: Ic05bdafacf06cde6a8d8dbae7512e5d22eb7f514
Signed-off-by: Depeng Shao <quic_depengs@quicinc.com>