RGB Matrix refactoring to open up for new drivers (#3913)

* rgb_matrix: use a driver ops struct

This is intended to avoid #ifdef proliferation on adding more drivers,
eg. model01, which use different architectures.

* rgb_matrix: document driver struct members

* rgb_matrix: remove unused LED testing code

* rgb_matrix: don't build into IS31x drivers unless being used

* rgb_matrix: refactor make config options

This ensures that the necessary files are included for any custom
RGB_MATRIX_ENABLE value, without having to add entries here for specific
boards. This particularly affects model01 because its controller is
integrated and won't be used anywhere else, so it's preferable not to
put it in common_features.mk.

This now validates the value of RGB_MATRIX_ENABLE.

It was necessary to fix an error in ergodox_ez rules.mk using the wrong
comment separator, yielding an invalid value.

* IS31x drivers: don't write the control registers all the time

This is only needed when they are changed. This is done in init() and
board- or keymap-specific code is free to make further changes.

* rgb_matrix: move structs from chip drivers to rgb_matrix_drivers.c

This approach is specific to the rgb_matrix functionality, so keep it
neatly separated from the raw chip drivers.

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