Move vp9_alloc_motion_field_info

Move vp9_alloc_motion_field_info out of init_tpl_buffer, so that
vp9_alloc_motion_field_info will be called even though there is
not alternate reference frame.

This fix the crash with shields_720p50 at bitrate 2000

Change-Id: If2877e8d0b8a834556be12d239b7b58ad1fc8c73
2 files changed