HDMI 4k - Use the TMDS maximum frequency to check mode dot clock (v3)

[Damien Lespiau]

This is the 3rd instance of the series to enable 4k displays. The previous
version was sent in August (oops):
  http://lists.x.org/archives/xorg-devel/2013-August/037520.html

Chris did catch a nice typo/bug that is fixed in this v3.

I also did not notice a question from Walter Harms (because he didn't Cc me, I
don't receive xorg-devel mails):
  http://lists.x.org/archives/xorg-devel/2013-August/037560.html 

> result->hdmi could be undefined, is that intentional ?
 
resut->hdmi cannot be undefined, it's initialized to NULL in
xf86MonitorFindHDMIBlock().

-- 
Damien