backends/drm: do one less atomic test per compositing cycle

As the compositor now always does the test, it's no longer necessary.

This should help reduce the impact that https://gitlab.freedesktop.org/drm/amd/-/issues/4358 has. Targeting 6.6 only because it's not that important for 6.5, and I'd like to be absolutely safe with this. It can perhaps be backported later on.