drm/tilcdc: Set up fbdev after fully registering device

Generic fbdev support is a DRM client. Set it up after fully registering
the new DRM device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Jyri Sarha <jsarha@ti.com>
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/20200408082641.590-8-tzimmermann@suse.de