modesetting: reorganise code into core and helper functions.

modesetting: reorganise code into core and helper functions.

This splits a lot of the core modesetting code out into a file of
helper functions, that are only called from themselves and/or the driver.

The driver gets called into more often or can call these functions from itself
if it is a helper using driver.

I've broken framebuffer resize doing this but I didn't like the API for that
in any case.