backends/drm: drop explicit modifiers completely if no output config works

Removing modifiers one by one creates a huge amount of combinations that
can make KWin hang for a long time in some cases. Instead of doing that,
leave it up to Mesa to pick a modifier when a setup doesn't work, which
will generally result in the least amount of bandwidth used and should be
able to power all output combinations.

src/backends/drm/drm_pipeline.cpp

@@ -504,12 +504,13 @@ bool DrmPipeline::pruneModifier()
         return false;
     }
     auto &modifiers = m_pending.formats[m_pending.layer->currentBuffer()->buffer()->format()];
-    if (modifiers.count() <= 1) {
+    if (modifiers.empty()) {
         return false;
-    }
-    modifiers.removeOne(m_pending.layer->currentBuffer()->buffer()->modifier());
+    } else {
+        modifiers.clear();
         return true;
     }
+}
 
 bool DrmPipeline::needsModeset() const
 {