Fixed use of motion percentage in KF/GF group calc

In both vp8_find_next_key_frame and define_gf_group,
motion_pct was initialised at the top of the loop before
next_frame stats had been read in.

This fix sets motion_pct after next_frame stats have
been read.

Change-Id: I8c0bebf372ef8aa97b97fd35b42973d1d831ee73