drm/mgag200: Separate device initialization into allocation

Embedding the DRM device instance in struct mga_device will require
changes to device allocation. Moving the device initialization into
its own functions gets it out of the way.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-12-tzimmermann@suse.de