[drm] Don't present on an output which is in dpms standby

It will fail if the output is not on, so let's don't try to present
on a disabled output.

backends/drm/drm_backend.cpp

@@ -557,6 +557,9 @@ bool DrmOutput::present(DrmBuffer *buffer)
         m_currentBuffer = buffer;
         return false;
     }
+    if (m_dpmsMode != DpmsMode::On) {
+        return false;
+    }
     if (m_currentBuffer) {
         return false;
     }