Force mode search on 64x64 if no mode is selected

Force mode search on 64x64 if no mode is selected

A speed feature disable_split_mask (set to 63) could cause no mode and
partition to be selected in rd_pick_partition because:

-> thresh_mult_sub8x8 all INT_MAX
-> All modes skipped for sub8x8 blocks
-> found_best_rd is 0 -> break from the loop of 4 sub blocks
-> sum_rdc is INT_MAX -> No rd update -> should_encode_sb is 0
-> Propagating to top of the tree
-> No partition / mode is selected

Bug: b/290499385
Change-Id: Ia655e262f3b32445347ae0aaf1a2d868cea997f3