Optimize get_txb_ctx()
Optimize get_txb_ctx()

Tested with 720p encoding and av1_cost_coeffs() saves 18% time.

Change-Id: If6de7c539c4b01a3066bdc267fb375dfe77c2c50

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