Declare setup_buffer_inter in vp9_rdopt.h

This funtion initializes buffer pointers and first stage motion vector
prediction. It will be needed by both regular rate-distortion
optimization loop and the non-RD mode decision. Hence move its
declaration in vp9_rdopt.h

Change-Id: I64e8b6316c9d05f20756a62721533a2e4d158235