fix: don't start HDMA in vblank

src/ppu.zig

@@ -569,6 +569,8 @@ pub const Ppu = struct {
         }
 
         // See if HBlank DMA is present and not enabled
+
+        if (!self.dispstat.vblank.read())
             pollBlankingDma(&cpu.bus, .HBlank);
 
         self.dispstat.hblank.set();