drm/ast: Initialize encoder and connector for VGA in helper function

Move encoder and connector initialization into a single helper and
put all related mode-setting structures into a single place. Done in
preparation of moving transmitter code into separate helpers. No
functional changes.

v2:
	* move encoder CRTC bitmask fix into separate patch (Javier)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-7-tzimmermann@suse.de