gl_shader_manager: Remove reliance on global accessor within MaxwellUniformData::SetFromRegs()

We can just pass in the Maxwell3D instance instead of going through the
system class to get at it.

This also lets us simplify the interface a little bit. Since we pass in
the Maxwell3D context now, we only really need to pass the shader stage
index value in.

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -320,7 +320,7 @@ void RasterizerOpenGL::SetupShaders(GLenum primitive_mode) {
         const std::size_t stage{index == 0 ? 0 : index - 1}; // Stage indices are 0 - 5
 
         GLShader::MaxwellUniformData ubo{};
-        ubo.SetFromRegs(gpu.state.shader_stages[stage]);
+        ubo.SetFromRegs(gpu, stage);
         const GLintptr offset = buffer_cache.UploadHostMemory(
             &ubo, sizeof(ubo), static_cast<std::size_t>(uniform_buffer_alignment));