drm/mgag200: Move DAC-register setup into model-specific code

Provide an init function for each model's DAC registers. Remove
the shared helper.

The code for initializing the DAC registers consisted of a large
table of default value, plus many exceptions for the various G200
models. Providing a per-model implementation makes if more readable.
At some point, some of the initialization should probably move into
the modesetting code.

v2:
	* don't duplicate DAC values unnecessarily (Sam, Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-3-tzimmermann@suse.de