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.
(cherry picked from commit d90c64bc)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com