codegen: Fix method pointer cast if instance isn't at first position
codegen: Fix method pointer cast if instance isn't at first position

By unconditionally putting the instance at first position the CCode
instance_pos attribute was not accounted for.

frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala