drm/mgag200: Inline mgag200_device_init()

Inline mgag200_device_init() into mgag200_device_create(), which is
the only caller. Also remove a duplicate error message for
mgag200_modeset_init(). The function will print an error if/where it
failed.

v2:
	* include a note about the removed error message in
	  changelog (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702075642.27834-3-tzimmermann@suse.de