Set adaptive_rd_thresh to 2 when simple block yrd is not used.

Fix regression caused by computing y sad without
int_pro_motion_estimation on low res motion clips.

Overall 0.07% quality loss on rtc_derf.

Change only affects low res on speed 8.

Change-Id: Ic6a188a56529f1034d6431005fb4b0e24e8a7e27