msm: camera: core: Destroy the context handle on shutdown

When the context is released, the handle is overwritten. As part of
the context shutdown sequence, save out the handle before releasing
it, and then explicitly destroy it.

Change-Id: I6ec81e7feb61f46cb0ffa27cedda26258857c5b9
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>