maxwell_3d: Initialize polygon modes

NVN expects this to be initialized as Fill, otherwise games that never
bind a rasterizer state will log an invalid polygon mode.

src/video_core/engines/maxwell_3d.cpp

@@ -107,6 +107,8 @@ void Maxwell3D::InitializeRegisterDefaults() {
     regs.rt_separate_frag_data = 1;
     regs.framebuffer_srgb = 1;
     regs.front_face = Maxwell3D::Regs::FrontFace::ClockWise;
+    regs.polygon_mode_back = Maxwell3D::Regs::PolygonMode::Fill;
+    regs.polygon_mode_front = Maxwell3D::Regs::PolygonMode::Fill;
 
     shadow_state = regs;