Your HD 620 graphics will dynamically allocate the memory needed, using upto half of your phyiscal memory.
You should not be messing with the registry. And, in case you are using the Intel generic drivers, you should use only the drivers provided by HP.
I agree with Al; there is absolutely no need - and no value proposition - for dedicating additional memory to video. The Intel HD Graphics drivers will take care of it automatically, allocating additional memory to graphics (up to 50%) as needs dictate.