backends/drm: work around llvmpipe implicit sync not working

implicit sync doesn't seem to work with glFlush + KMS and llvmpipe doesn't support EGL_ANDROID_native_fence_sync either, so we need to wait for rendering to complete in a blocking fashion.

I'd like to wait with merging this until it actually becomes necessary though; hopefully this can be fixed on the Mesa side before Plasma 6 (https://gitlab.freedesktop.org/mesa/mesa/-/issues/9375)