107de196 changes the encoder alt-ref selection behavior. Assuming
min_gf_interval = max_gf_interval = 4, the frame order would be
frm_1  arf_1  frm_2  frm_3  frm_4  frm_5  arf_2 before 107de196;
frm_1  arf_1  frm_2  frm_3  frm_4  arf_2  frm_5 after 107de196.

This patch reverts such alt-ref placement change.

Change-Id: I93a4a65036575151286f004d455d4fcea88a1550