regs_shader: Intialize GPUREG_SH_INPUTBUFFER_CONFIG to default value

* Default value verified on hw. Tales of Abyss does not update the number of vertex attributes for the geometry unit and expects it to be 2

src/video_core/pica/regs_shader.h

@@ -35,6 +35,7 @@ struct ShaderRegs {
 
     union {
         // Number of input attributes to shader unit - 1
+        u32 raw{0xa0000001};
         BitField<0, 4, u32> max_input_attribute_index;
         BitField<8, 8, u32> input_to_uniform;
         BitField<24, 8, ShaderMode> shader_mode;