vp8cx_mb_init_quantizer was being called for every mode checked
in vp8_rd_pick_inter_mode.  zbin_extra is the only value that
really needs to be recalculated.  This calculation is disabled
when using the fast quantizer for mode selection.
This gave a small performance boost (~.5% to 1%).
Note: This needs to be verified with segmentation_enabled.

Change-Id: I62716a870b3c82b4a998bdf95130ff0b02106f1e