drm/virtio: fix an error code in virtio_gpu_init()

If devm_request_mem_region() fails this code currently returns success
but it should return -EBUSY.

Fixes: 6076a971

 ("drm/virtio: implement blob resources: probe for host visible region")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/YBpy0GS7GfmafMfe@mwanda
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>