backends/drm: remove unnecessary comment

DrmPipeline::needsModeset, and by extension, DrmGpu::needsModeset is no
longer expensive in any way, so checking them multiple times per frame is
not a problem anymore

src/backends/drm/drm_pipeline.cpp

@@ -58,7 +58,6 @@ DrmPipeline::~DrmPipeline()
 
 bool DrmPipeline::testScanout()
 {
-    // TODO make the modeset check only be tested at most once per scanout cycle
     if (gpu()->needsModeset()) {
         return false;
     }