drm/mgag200: Call mgag200_driver_{load, unload}() before registering device

The load/unload callbacks in struct drm_driver are deprecated. Remove
them and call functions explicitly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205090252.5711-1-tzimmermann@suse.de