Depth 30 enablement for modesetting-ddx and fixups for glamor. Rev 2

[Mario Kleiner]

2nd revision of the server/modesetting-ddx/glamor patches for depth 30,
following Eric's and Michel's suggestions (thanks!)..

These ones tested on nouveau, amd, intel with the modesetting ddx + glamor
and also (for glamor again) against the latest ati-ddx and amdgpu-ddx,
which now can use depth 30 under glamor.

-mario