drm/mgag200: Move PLL setup out of mode-setting function

Makes the code slightly more flexible. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-3-tzimmermann@suse.de