Speed up ext-intra inter frame encoding

Skip filter intra mode search when regular intra modes have large
rd cost.

Encoding speed improvement:  8%.

Compression performance drop: 0.02%  / 0.09%  / 0.03% on
                              lowres / midres / hdres

Change-Id: I94d3e48781bff6ae6895a54f271dd65c959bb976