msm: vidc: Clean session immediately after session end

After driver has received session_end_done from firmware,
it no longer needs to retain the session information, since
firmware has already cleaned up the session. If the driver
sends any further session command to firmware, there will
not be any valid response from firmware. Ensure this by cleaning
up the session as soon as session end is received.

CRs-Fixed: 993591
Change-Id: Ia7517c0e6beab1178d70b72b3f5c0b512b400bd8
Signed-off-by: Arun Menon <avmenon@codeaurora.org>