drm/virtio: document drm_dev_set_unique workaround

A while back we removed it, yet that lead to regressions. At some later
point, I've attempted to remove it again without fully grasping the
unique (pun intended) situation that virtio is in.

Add a bulky comment to document why the call should stay as-is, for the
next person who's around.

As a Tl;Dr: virtio sits on top of struct virtio_device, which confuses
dev_is_pci(), wrong info gets sent to userspace and X doesn't start.
Driver needs to explicitly call drm_dev_set_unique() to keep it working.

v2: Fix handful of typos (Laszlo)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181024144252.16518-1-emil.l.velikov@gmail.com