drm/mgag200: Set up fbdev after registering device; remove error checks

Generic fbdev support is a DRM client. Set it up after registering
the new DRM device. Remove the error checks as the driver's probe
function should not depend on a DRM client's state.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408082641.590-7-tzimmermann@suse.de