Depth 30 enablement for modesetting-ddx and fixups for glamor. Rev 3

[Mario Kleiner]

Revision 3, with r-b's by Antoine Martin tacked on, thanks. Antoine, i
left that one /* XXX handle 2 10 10 10 and 1555 formats; ... */ comment
in place. As far as i understand it is meant to be a todo for depth 30 and
depth 16 formats with alpha channel, and the current code only handles
10 bpc and 5 bpc formats without alpha channel, so that todo would
still apply.

Also updated to address Ville's and Michel's comments.

thanks,
-mario