Enable reduced set of intra modes in rtc coding

Enable reduced set of intra modes in rtc coding

This commit enables the use of DC, vertical, and horizontal intra
prediction mode in rtc non-RD mode decision. When the best cost value
of inter modes is above a given threshold, the encoder runs the
above three intra modes and selects the one that has minimum
prediction residual in terms of SAD.

This together with recent changes on non-RD mode decision and coding
control improves compression performance of speed -6 by
derf  91%
yt    61%
hd    46%
stdhd 52%

In terms of encoding speed, it is about 3 times faster than speed -5.

Change-Id: I6b483bfd0307e6482bb22a6676ae4e25a52b1310