Fixing sub-optimal rdloop when testing supertx on 8x4/4x8 blocks

Remove early termination in vp9_rd_pick_inter_mode_sub8x8() in
order to complete mode selection for 8x4/4x8 blocks which will
try supertx in a higher level function.

Change-Id: I457505257332f70d9cd8d22db52ad32ff15f7f87