drm/aspeed: Use devm_drm_dev_alloc

As usual, we can drop the drm_dev_put() and need to embed the
drm_device. Since it's so few, also go right ahead and leave
drm_device->dev_private set to NULL, so that we always use the
container_of() upcast, which is faster anyway.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Andrew Jeffery <andrew@aj.id.au>
Cc: linux-aspeed@lists.ozlabs.org
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-56-daniel.vetter@ffwll.ch