Only do uv intra-mode evaluation when intra mode is checked

When we encode slide-show clips, for the majority of the time,
only ZEROMV mode is checked, and all other modes are skipped.
This change delayed uv intra-mode evaluation until intra mode is
actually checked. This gave big performance gain for slide-show
video encoding (2nd pass gain: 18% to 28%). But, this change
doesn't help other types of videos.

Also, zbin_mode_boost is adjusted in mode-checking loop, which
causes bitstream mismatch before/after this change when --best
or --good with --cpu-used=0 are used.

Change-Id: I582b3e69fd384039994360e870e6e059c36a64cc