Segmentation feature logic fix.

Bug fix to logic in vp8_pick_inter_mode() and
vp8_rd_pick_inter_mode().

The block on the use of segment features
for the cm->refresh_alt_ref_frame case
was just for testing and is not correct.

The special case code for alt ref can
be re-enabled as an else clause.

Change-Id: Ic9b57cdb5f04ea7737032b8fb953d84d7717b3ce