drm/mgag200: Provide per-device callbacks for PIXPLLC

Move the PIXPLLC code into per-model source files and wire it up
with per-model callbacks. No functional changes.

The PIXPLLC pixel-clock is part of the CRTC, but really separate
hardware that varies with each model of the G200. Move the PIXPLLC
code for each model into the per-model source file and call it from
CRTC helpers via device functions.

This allows to remove struct mgag200_pll and the related code. The
new callbacks behave like the CRTC's atomic_check and atomic_enable
functions.

v3:
	* clean up style

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-12-tzimmermann@suse.de