Revert "glamor: Avoid glamor_create_pixmap for pixmaps backing windows"

This reverts commit 27470308.

Reports of visual corruption were bisected to this, e.g.
https://bugs.archlinux.org/task/61941

 . I can reproduce this with Turks,
but not with Bonaire. I assume it's a Mesa/glamor bug, but let's revert
for now.

Acked-by: Alex Deucher <alexander.deucher@amd.com>