Use devm_drm_dev_init() and drop using tinydrm_device.

v2: devm_drm_dev_register() was dropped so add driver release callbacks.
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225144232.20761-6-noralf@tronnes.org