gbm: add missing R16 case in gbm_bo_get_bpp
This ensures that gbm_bo_get_bpp now returns 16 instead of 0 for buffers with GBM_FORMAT_R16. R16 was the only non-YUV format from gbm.h that was missing.
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This ensures that gbm_bo_get_bpp now returns 16 instead of 0 for buffers with GBM_FORMAT_R16. R16 was the only non-YUV format from gbm.h that was missing.