Calling a spring election is not that easy, there are only two paths to do it.
1) Change the law - sort of a giveaway as to the PMs intentions
2) Get the House of Commons to vote non-confidence in the PM - politically stupid when you have a majority and need your own party to vote against you
The idea the the PM could go to the GG and ask for an early election ignores the fact that the parliament has created a law stating when the election should be held. If the GG does as the PM wants he is on contravening the desire of parliament.
When the government has a minority is it legitimate for the PM to ask for an early election because it is clear to the GG that the PM does not have the confidence of the house.
So with a majority it is not easy to call an early election. If the PM were to try and ask for one and the GG refused we would have a constitutional crisis.