Revert "backends/drm: use glReadPixels for CPU copy"

This reverts commit fdbcb184. It didn't
take into account that the framebuffer y axis is inverted in comparison
to git master, so using glReadPixels causes the image to be inverted in 5.27.
Fixing that by always copying into an intermediary buffer is possible, but
would also negate most or all of the performance improvements, so reverting
the commit is the best way to deal with this issue.

BUG: 472142