backends/drm: don't set the dpms mode to AboutToTurnOff if the screen is already off
Otherwise, DrmOutput::setDrmDpmsMode will inhibit the renderloop for a second time, and it'll never be completely un-inhibited, which means the screen stays off.
Otherwise, DrmOutput::setDrmDpmsMode will inhibit the renderloop for a second time, and it'll never be completely un-inhibited, which means the screen stays off.