When segment feature is on, frame level cm->tx_mode can be set to
ONLY_4X4 only if all segments are lossless. Otherwise will cause
bugs when xd->lossless[i] is 0 and xd->lossless[0] is 1.
Also fix the condition of coding tx_type, which should be on when
the qindex of current segment is > 0.

BUG=aomedia:106
BUG=aomedia:104

Change-Id: Ic076083bb78b3b99a6f7d17ec82ee402c64bcc52