drm/virtio: remove irrelevant DRM_UNLOCKED flag

DRM_UNLOCKED doesn't do anything for non-legacy drivers. Remove it.

Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190522154702.16269-2-emil.l.velikov@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>