Increase entropy_context buffer size in sub8x8 RD

Temporarily increase the buffer size from 2 to 4 in the sub8x8
rate-distortion search for cb4x4. These functions will be deprecated
after cb4x4 is fully enabled.

Change-Id: I6aecaf55bd21a9416713cf31e75e7b4d9b68f83c
1 file changed