backends/drm: don't assume we shouldn't use modifiers without IN_FORMATS

Instead, use a linear modifier

src/backends/drm/drm_object_plane.cpp

@@ -99,7 +99,7 @@ bool DrmPlane::init()
             }
         } else {
             for (uint32_t i = 0; i < p->count_formats; i++) {
-                m_supportedFormats.insert(p->formats[i], {});
+                m_supportedFormats.insert(p->formats[i], {DRM_FORMAT_MOD_LINEAR});
             }
         }
         if (m_supportedFormats.isEmpty()) {