Chris Willman Music WriterWith due respect to Joni Mitchell’s lyrical prowess, never were there less prophetic words than “Don’t it always seem to go that you don’t know what you’ve got till it’s gone” — if only for the due that Mitchell herself is receiving while she is around to take it in. The queen of singer-songwriters got what may be the best of all possible salutes Friday night, feted and serenaded in a three-hour tribute concert that capped MusiCares’ annual pre-Grammy MusiCares fundraising dinner.And even though the event, like the Grammy telecast itself, was transported to Las Vegas this year, with the main ballroom at the MGM Grand Conference Center sitting in for MusiCares’ usual L.A.