msm: vidc: Add null to context bank mapping pointer after release

- Issue: There is a crash happening at the kernel if the context bank
         release function is called subsequently.

- Root cause: The context bank mapping pointer is pointing to invalid
              address once the context banks has been released and
			  hence on subsequently calling the context bank release
			  function, it is trying to access invalid address.

- Fix: Make the context bank mapping pointer to null once the
       context banks has been released.

Change-Id: I4074bc33db05fbc8bde4fcdb82bc4391c8f7fee3
Signed-off-by: Sayantan Majumder <smajumde@codeaurora.org>
Signed-off-by: UtsavBalar1231 <utsavbalar1231@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>