drm/mgag200: Abstract pixel PLL via struct mgag200_pll

Move all PLL compute and update functions into mgag200_pll.c. No
functional changes to the rsp algorithms.

Introduce struct mgag200_pll and mgag200_pll_funcs. The data strutures
abstract the details of each revision's PLL. Perform calls to compute
and update functionality via function pointers. Init the PLL once as
part of the driver initialization.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-11-tzimmermann@suse.de