virtio-gpu: fix memory leak in update_cursor_data_virgl

In update_cursor_data_virgl function, if the 'width'/ 'height'
is not equal to current cursor's width/height it will return
without free the 'data' allocated previously. This will lead
a memory leak issue. This patch fix this issue.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-id: 58187760.41d71c0a.cca75.4cb9@mx.google.com
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
