backends/drm: only enforce modifiers default for surface creation

Whether or not we want to use explicit modifiers for our surfaces doesn't
matter for what format+modifiers drm planes support. This way direct scanout
works by default, without having to explicitly enable modifiers

src/backends/drm/drm_object_plane.cpp

@@ -73,10 +73,7 @@ bool DrmPlane::init()
         checkSupport(5, Transformation::ReflectY);
 
         // read formats from blob if available and if modifiers are supported, and from the plane object if not
-        bool modifiersEnvSet = false;
-        bool modifiersEnv = qEnvironmentVariableIntValue("KWIN_DRM_USE_MODIFIERS", &modifiersEnvSet) != 0;
-        bool allowModifiers = (gpu()->isNVidia() && !modifiersEnvSet) || (modifiersEnvSet && modifiersEnv);
-        if (auto formatProp = getProp(PropertyIndex::In_Formats); formatProp && gpu()->addFB2ModifiersSupported() && allowModifiers) {
+        if (auto formatProp = getProp(PropertyIndex::In_Formats); formatProp && gpu()->addFB2ModifiersSupported()) {
             DrmScopedPointer<drmModePropertyBlobRes> propertyBlob(drmModeGetPropertyBlob(gpu()->fd(), formatProp->current()));
             if (propertyBlob && propertyBlob->data) {
                 auto blob = static_cast<drm_format_modifier_blob*>(propertyBlob->data);

src/backends/drm/egl_gbm_backend.cpp

@@ -152,11 +152,14 @@ bool EglGbmBackend::resetOutput(Output &output)
     QVector<uint64_t> modifiers = output.output->supportedModifiers(m_gbmFormat);
 
     QSharedPointer<GbmSurface> gbmSurface;
+    bool modifiersEnvSet = false;
+    static bool modifiersEnv = qEnvironmentVariableIntValue("KWIN_DRM_USE_MODIFIERS", &modifiersEnvSet) != 0;
+    static bool allowModifiers = (gpu()->isNVidia() && !modifiersEnvSet) || (modifiersEnvSet && modifiersEnv);
 #if HAVE_GBM_BO_GET_FD_FOR_PLANE
-    if (modifiers.isEmpty()) {
+    if (!allowModifiers) {
 #else
     // modifiers have to be disabled with multi-gpu if gbm_bo_get_fd_for_plane is not available
-    if (modifiers.isEmpty() || output.output->gpu() != m_gpu) {
+    if (!allowModifiers || output.output->gpu() != m_gpu) {
 #endif
         int flags = GBM_BO_USE_RENDERING;
         if (output.output->gpu() == m_gpu) {