broadcom/vc5: Emit proper flatshading code for glShadeModel(GL_FLAT).

In updating the simulator, behavior changed slightly so that our old code
wasn't getting glxgears's flatshading interpolated right.  Emit flat
shading code just like we would for a normal flat-shaded varying, by
passing a flag in the shader key for glShadeModel(GL_FLAT) state and
customizing the color inputs based on that.

src/broadcom/compiler/vir.c

@@ -716,8 +716,6 @@ v3d_set_fs_prog_data_inputs(struct v3d_compile *c,
 
         memcpy(prog_data->flat_shade_flags, c->flat_shade_flags,
                sizeof(c->flat_shade_flags));
-        memcpy(prog_data->shade_model_flags, c->shade_model_flags,
-               sizeof(c->shade_model_flags));
 }
 
 uint64_t *v3d_compile_fs(const struct v3d_compiler *compiler,