Processing ADC conversion result: DMA vs Processor Registers
If you just write a function that reads the ADC result into a variable, does some math on it, and then writes the result to PWM, and if you use a decent compiler with optimization turned up, then the critical data will probably end up in a register whether you want it to or not.  That's really not that much different from setting up DMA just so that you can fetch one memory location and do the same thing.

Where I would use DMA for this sort of thing is if I were going to do something like fetch multiple ADC samples per cycle of the controller (which I often do) -- the DMA saves a lot vs. the overhead of an ISR to collect the data from an ADC.