habanalabs: add handle field to context structure

This patch adds a field to the context's structure that will hold a unique
handle for the context.

This will be needed when the user will create the context.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>