msm: vidc: Fix session error handling

Do not clean the session while handling session error, instead
only set the state to invalid and report the error to client.
The session will be cleaned as part of kill_session called when
clients calls flush/stop. Also ignore all prepare_buf calls when
in invalid state.

CRs-Fixed: 748031
Change-Id: I2d8824cddb6f86b3feb42e6021f1e0abad26be3e
Signed-off-by: Shalaj Jain <shalajj@codeaurora.org>