drm/virtio: fix framebuffer sparse warning

drm/virtio: fix framebuffer sparse warning

[ Upstream commit 71d3f6ef7f5af38dea2975ec5715c88bae92e92d ]

virtio uses normal ram as backing storage for the framebuffer, so we
should assign the address to new screen_buffer (added by commit
17a7b0b4d9749f80d365d7baff5dec2f54b0e992) instead of screen_base.

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>