drm/mgag200: Use managed mode-config initialization

Mode configuration is now cleanued up automatically. While at it,
move all mode-config code into mgag200_mode.c. Done in preparation
of switching mgag200 to simple-KMS helpers.

v2:
	* improve commit message (Sam)
	* rebased during cherry pick
	* also move bpp_shift initialization

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507090315.21274-5-tzimmermann@suse.de