libplacebo: utils: fix comparison warnings

../../modules/video_output/libplacebo/utils.c: In function ‘vlc_placebo_DoviMetadata’:
    ../../modules/video_output/libplacebo/utils.c:472:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      472 |     for (int c = 0; c < ARRAY_SIZE(dst->comp); c++) {
          |                       ^
    ../../modules/video_output/libplacebo/utils.c:486:35: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      486 |                 for (int k = 0; k < ARRAY_SIZE(cdst->poly_coeffs[i]); k++) {
          |                                   ^
    ../../modules/video_output/libplacebo/utils.c:496:39: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      496 |                     for (int k = 0; k < ARRAY_SIZE(cdst->mmr_coeffs[i][j]); k++)
          |                                       ^