Fix a bug in vp9_rd_pick_inter_mode_sb
Fix a bug in vp9_rd_pick_inter_mode_sb

This commit fixes a bug related to skipping intra mode checking, by
using a separate variable to store the best prediction error from
inter mode. It avoids unintentionally overwriting intra mode
rate-distortion cost, and hence affecting other speed features.

Change-Id: I99e12993339c84c8b4f597996b372012e5858fae

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