Make sure MadVR is installed, then while a movie is playing with MPC-HC, right click on the MadVR icon in the taskbar (or right-cligh on the screen and select filters/MadVR/edit settings in MPC-HC). This should bring up MadVR's settings.
Find your display device (under device), click on the little arrow to reveal the options for that device, then click display modes. You should see a blank line under "list all display modes MadVR may switch to". Enter the display modes you want MadVR to switch to (they have to be supported by your display under windows) following the template given as an example:
The refresh rate follows the resolution, and windows refresh rates are used, for example 23 stands for 23.976hz, 24 stands for 24.00hz, etc.
1080p23, 1080p24, 1080p50, 1080p59
Select ok/apply, and all should work as before...