drm/ast: Split ast_set_ext_reg() into color and threshold function

In ast_set_ext_reg() sets several framebuffer options and CRT threshold
parameters. The former is mostly state of the primary plane; the latter
is constant. Hence, split the function in two and make it work with
atomic modesetting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107083404.6852-5-tzimmermann@suse.de