[drm] Do not blank crtc in atomic mode

In atomic setting mode, when external display tears down, it will blank
crtc with dump buffer by legacy api, then re-plug the external display,
the blank can not be removed. We think the blank is not needed in atomic
mode setting.

Signed-off-by: Chaojiang Luo <chaojiangluo@163.com>

plugins/platforms/drm/drm_object_crtc.cpp

@@ -91,6 +91,11 @@ bool DrmCrtc::blank()
     if (!m_output) {
         return false;
     }
+
+    if (m_backend->atomicModeSetting()) {
+        return false;
+    }
+
     if (!m_blackBuffer) {
         DrmDumbBuffer *blackBuffer = m_backend->createBuffer(m_output->pixelSize());
         if (!blackBuffer->map()) {