fix #1506: film grain SEI encoder use wrong bits to signal num_model_values_minus1

Taoran Lu requested to merge tlu/HM:fix_filmgrain_sei_encoder into master

The syntax element num_model_values_minus1 should be signaled with 3 bits according to spec. The encoder SEI function xWriteSEIFilmGrainCharacteristics() mistakenly uses 8 bits. The decoder parsing is correct.

Edited by Taoran Lu

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