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

[Mario Kleiner]

These patches against master teach the modesetting ddx color depth 30
support. They also fix a few loose ends in the new glamor depth 30
enablement, which caused psychedelic colors under desktop composition
on all gpus depending on the color depth on the different windows -
some worked fine, some others not.

Successfully tested so far on KDE-5 without compositing, with XRender
based compositing, and with OpenGL compositing under GLX and EGL.
I used my photometer to make sure the bits come through while testing
on NVidia + nouveau, and then also quickly tested on old AMD gfx +
radeon-kms and on Intel + intel-kms to make sure that regular desktop
and OpenGL apps render correctly on that hw as well.

The included bypass for color palettes / xf86HandleColormaps at depth 30,
which was meant to fix some random crashes at server startup or shutdown,
also showed that the hw gamma luts on intel and nvidia do not get bypassed
in 30 bit mode by the hw, as i thought and stated before. That "bypass"
was just a side-effect of silent failures in the servers colormap handling.

thanks,
-mario