Modify UBL mesh_is_valid and use in leveling_is_valid (#10746)

Marlin/src/feature/bedlevel/bedlevel.cpp

     #elif ENABLED(AUTO_BED_LEVELING_BILINEAR)
       !!bilinear_grid_spacing[X_AXIS]
     #elif ENABLED(AUTO_BED_LEVELING_UBL)
-      true
+      ubl.mesh_is_valid()
     #else // 3POINT, LINEAR
       true
     #endif

Marlin/src/feature/bedlevel/ubl/ubl.h

       static void line_to_destination_cartesian(const float &fr, const uint8_t e);
     #endif
 
-    #define _CMPZ(a,b) (z_values[a][b] == z_values[a][b+1])
-    #define CMPZ(a) (_CMPZ(a, 0) && _CMPZ(a, 1))
-    #define ZZER(a) (z_values[a][0] == 0)
-
-    FORCE_INLINE bool mesh_is_valid() {
-      return !(
-        (    CMPZ(0) && CMPZ(1) && CMPZ(2) // adjacent z values all equal?
-          && ZZER(0) && ZZER(1) && ZZER(2) // all zero at the edge?
-        )
-        || isnan(z_values[0][0])
-      );
+    inline static bool mesh_is_valid() {
+      for (uint8_t x = 0; x < GRID_MAX_POINTS_X; x++)
+        for (uint8_t y = 0; y < GRID_MAX_POINTS_Y; y++)
+          if (isnan(z_values[x][y])) return false;
+      return true;
     }
 
 }; // class unified_bed_leveling

Marlin/src/gcode/bedlevel/M420.cpp

     // L or V display the map info
     if (parser.seen('L') || parser.seen('V')) {
       ubl.display_map(parser.byteval('T'));
-      SERIAL_ECHOLNPAIR("ubl.mesh_is_valid = ", ubl.mesh_is_valid());
-      SERIAL_ECHOLNPAIR("ubl.storage_slot = ", ubl.storage_slot);
+      SERIAL_ECHOPGM("Mesh is ");
+      if (!ubl.mesh_is_valid()) SERIAL_ECHOPGM("in");
+      SERIAL_ECHOLNPAIR("valid\nStorage slot: ", ubl.storage_slot);
     }
 
   #endif // AUTO_BED_LEVELING_UBL