Fixed this_mode used before set in vp8_pick_inter_mode

The variable this_mode was being used before it had been
initialized.

Moved the line that sets-up this_mode toward the top of the
enclosing loop, prior to its first use. The bug would result in
tests in the loop lagging the mode that was expected to be
tested.

Change-Id: If4e51600449ce6b4285f112da17a44c24b4a19fb