drm/ast: Initialize mode setting in ast_mode_config_init()

There's modesetting init code in ast_main.c. Move it to ast_mode.c and
merge it with the modesetting init code in ast_mode_init(). The result
is ast_mode_config_init(), which initalizes the whole modesetting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-15-tzimmermann@suse.de