backends/drm: record GPU render time

This should make it much less likely for KWin to miss frames.

Note that this is still far from perfect, without !2425 (closed) rendering times are still heavily dependent on clients. For example, I see 60-70 dropped frames per second with Jedi Fallen Order on my 120Hz screen.

This also doesn't measure direct scanout, but from my measurements it takes about 40-100µs, so it can be safely ignored.

cc #124

closes #47 (closed)