msm: camera: isp: use proper buffer manager lock

Buffer manager is shared by 2 VFE, so buffer operation
need to acuire lock on buf mgr instead of on each vfe dev.

Use original buffer manager lock to protect instead of
new per vfe device buf manager lock.

Change-Id: I39a4f31b8769b867850babd29b64f785bcbc4374
Signed-off-by: Peter Liu <pingchie@codeaurora.org>