backends/drm: fix multi gpu with NVidia as primary

NVidia GPUs can't render to linear buffers, so we can't force a linear modifier for CPU rendering. The proprietary NVidia driver also can't map gbm buffers with all formats and modifiers, so instead of mapping the gbm buffer, this patch changes KWin to instead use glReadPixels for accessing the source buffer

Using glReadPixels also seems to have a lower CPU overhead than the gbm map path, so I'd like to backport that part to 5.27