I also did a full review of all callers, and only the xen driver
forgot to call drm_dev_put in the failure path. Fix that up too.

v2: I noticed that xen has a drm_driver.release hook, and uses
drm_dev_alloc(). We need to remove the kfree from
xen_drm_drv_release().

bochs also has a release hook, but leaked the drm_device ever since

commit 0a6659bd
Author: Gerd Hoffmann <kraxel@redhat.com>
Date:   Tue Dec 17 18:04:46 2013 +0100

    drm/bochs: new driver

This patch here fixes that leak.

Same for virtio, started leaking with

commit b1df3a2b
Author: Gerd Hoffmann <kraxel@redhat.com>
Date:   Tue Feb 11 14:58:04 2020 +0100

    drm/virtio: add drm_driver.release callback.
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-5-daniel.vetter@ffwll.ch