x->blk_skip used to be uninitialzied (leftover from encoding the
previous block), if cm->tx_mode != TX_MODE_SELECT (which is used with
higher --cpu-used or --rt options). This resulted in degraded coding
performance when uning cm->tx_mode != TX_MODE_SELECT.

This fixes the VP10/EndToEndTestLarge.EndtoEndPSNRTest/40 unit test.

Change-Id: If39062927446798c626fc93694b4e6a4f35fa5da