libplacebo: display: use array for planes
vlc_placebo_PlaneData is expecting an array of four elements. Not complying to this results in the following warning, even if the code is not making use of the other planes.
../../modules/video_output/libplacebo/display.c:369:18: warning: ‘vlc_placebo_PlaneData’ accessing 480 bytes in a region of size 120 [-Wstringop-overflow=]
369 | if (!vlc_placebo_PlaneData(r->p_picture, &subdata, NULL))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~