msm: camera: memmgr: reduce mutex lock duration

Allow concurrent calls to unmap buffer by reducing the
mutex lock duration, and allow same fd for mapping user iova,
use book keeping for user smmu.

CRs-Fixed: 2559879
Change-Id: Ic3c92d4fa0ca7d1c29e621172e0b058420e1501d
Signed-off-by: Haibin Liu <haibinl@codeaurora.org>