drm/mgag200: Split PLL compute function for G200SE by rev

The compute function for G200SE pixel PLLs handles two revisions with
different algorithms. Split it accordingly to make it readable. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714142240.21979-9-tzimmermann@suse.de