drm/mgag200: Move device init and cleanup to mgag200_drv.c

Moving the initializer and cleanup functions for device instances
to mgag200_drv.c prepares for the conversion to managed code. No
functional changes are made. Remove mgag200_main.c, which is now
empty.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-11-tzimmermann@suse.de