backends/drm: don't set gamma with legacy unless really necessary

It seems to have a pretty large performance impact

src/backends/drm/drm_pipeline.h

@@ -168,6 +168,7 @@ private:
 
     bool m_modesetPresentPending = false;
     bool m_didLegacyScanoutHack = false;
+    std::shared_ptr<DrmGammaRamp> m_currentLegacyGamma;
 
     struct State
     {

src/backends/drm/drm_pipeline_legacy.cpp

@@ -132,9 +132,12 @@ DrmPipeline::Error DrmPipeline::applyPendingChangesLegacy()
                 return err;
             }
         }
-        if (m_pending.gamma && drmModeCrtcSetGamma(gpu()->fd(), m_pending.crtc->id(), m_pending.gamma->lut().size(), m_pending.gamma->lut().red(), m_pending.gamma->lut().green(), m_pending.gamma->lut().blue()) != 0) {
-            qCWarning(KWIN_DRM) << "Setting gamma failed!" << strerror(errno);
-            return errnoToError();
+        if (m_pending.gamma && m_currentLegacyGamma != m_pending.gamma) {
+            if (drmModeCrtcSetGamma(gpu()->fd(), m_pending.crtc->id(), m_pending.gamma->lut().size(), m_pending.gamma->lut().red(), m_pending.gamma->lut().green(), m_pending.gamma->lut().blue()) != 0) {
+                qCWarning(KWIN_DRM) << "Setting gamma failed!" << strerror(errno);
+                return errnoToError();
+            }
+            m_currentLegacyGamma = m_pending.gamma;
         }
         if (m_connector->contentType.isValid()) {
             m_connector->contentType.setEnumLegacy(m_pending.contentType);