drm/ingenic: Don't set drm_device->dev_private

Entirely not used, just copypasta.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-32-daniel.vetter@ffwll.ch