Refactor buf_offset in  av1_inv_txfm2d.c
Refactor buf_offset in  av1_inv_txfm2d.c

Change-Id: I73d1d15ab678242737432064d203c476057286ed

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom