msm: camera: Fix invalid access of vb2 buffer

vb2 buffers in camera generic buffer manager should
be accessed only in get buff on success. In other calls
we should not access without validation of the buffer
which is done only as part of msm vb2 driver.

CRs-Fixed: 2007389
Change-Id: Iae57ca1389ebc5ffd0a4a88e132ad30b205f7c9e
Signed-off-by: Hariram Purushothaman <hariramp@codeaurora.org>