msm: camera: mem: Allow buffer queue to be reset at the time of unmapping

mutex unlock is not happening at the time of error case when kmd access
dmabuf unmapping is not successful. This change fixes this issue and
allowing buffer index to be reset it.

Change-Id: I6aad9e189bb7e7949742ddf1e018e6eeb10cd164
Signed-off-by: Jigarkumar Zala <jzala@codeaurora.org>