drm/ast: Clean up arguments of register functions

In preparation of an update of the primary-plane code, this patch
changes the arguments of several register-setting functions. No
functional changes are made. The function ast_set_dac_reg() has no
effect and therefore gets removed entirely.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202111557.15176-4-tzimmermann@suse.de