vp8_find_near_mvs() is being called on all possible reference frames
but the data computed may be used if the loop exits early, which can
be due to x->skip beign set to 1.

Optimize this by call vp8_find_near_mvs() laziy only if it is going
to be used and not computed yet.

Change-Id: Iccdbd4c962a670c9f2c99b8aca8096042ca5dc98