Not to call reset_use_upsampled_references() while encoding key frames

A crash was reported while encoding >1080p file using best mode.
(aom issue 107 and issue 108). The cause was that the speed features
were initialized using good mode in AV1. The patch(commit f330ca)
disabled the upsampled reference usage only for good mode, but not for
best mode. So, for 1st key frame, the encoder incorrectly called
reset_use_upsampled_references() that caused the segfault.

The patch(commit e4e93b) disabled the upsampled reference usage for all
encoding modes and resolved the issue. This patch fixed issue 108 and
further made sure the upsampled references would be used correctly.

BUG=aomedia:108

Change-Id: I8362cc7cd82be324fd77f0bec315aeba509903bd