maxwell_dma: Rename registers to match official docs and reorder

Rename registers in the MaxwellDMA class to match Nvidia's official
documentation. This one can be found here:

https://github.com/NVIDIA/open-gpu-doc/blob/master/classes/dma-copy/clb0b5.h

While we are at it, reorganize the code in MaxwellDMA to be separated in
different functions.