wayland-dma-buf: Fix 32bpp channel order inversion

Apparently my understanding of Cogl pixel formats, or at least their
use, was somewhat shaky.

Un-invert the inversion of the DRM FourCC -> Cogl pixel format mapping
when creating dmabufs from clients, fixing inverted channel ordering
seen from GL clients, e.g. gold highlights in gtk4-demo when using the
GSK GL backend when they should be blue.

https://bugzilla.gnome.org/show_bug.cgi?id=786677