msm: vidc: Handle race condition for accessing session head

During CVP internal session, CVP close done calls for session
clean. session_clean API acquires the device lock and destroys
the cvp hal_session. At the same time, if decoder session is
also in the verge of close, since session_end does not have a
device lock, there is high chance to access corrupted hal_session
values.

Change-Id: I34593e6507da9bad13c6d92faf40c4d790825d39
Signed-off-by: Priyanka Gujjula <pgujjula@codeaurora.org>