The DPM state machine functions including dpm_start() are part of the stack and defined in the pre-compiled library so the source isnt available in the SDK.
Please refer to the summary of the functions in dpm.h header file as well as Table 18 in the CCGx_FW_UserGuide. The dpm_start() is called in main() and is used to start the PD state machine for the port.
I couldnt understand what you meant by "The function is responsible for this dpm_start()"? To disable or enable your MOSFETs, you could use the vbus_fet_on() and vbus_fet_off() functions defined in psource.c file, which are called based on the app callback received from the DPM stack.