backends/drm: only use atomic gamma size with AMS

For some reason the property exists with legacy, too. If you try to set
legacy gamma with atomic gamma size it'll fail

src/backends/drm/drm_object_crtc.cpp

@@ -86,10 +86,12 @@ void DrmCrtc::setNext(const QSharedPointer<DrmBuffer> &buffer)
 
 int DrmCrtc::gammaRampSize() const
 {
+    if (gpu()->atomicModeSetting()) {
         // limit atomic gamma ramp to 4096 to work around https://gitlab.freedesktop.org/drm/intel/-/issues/3916
         if (auto prop = getProp(PropertyIndex::Gamma_LUT_Size); prop && prop->current() <= 4096) {
             return prop->current();
         }
+    }
     return m_crtc->gamma_size;
 }