The feature that sets the amount of memory that is used for video is called Intel® Dynamic Video Memory Technology and this will make the graphics controller only to use the amount of RAM memory that it requires to run the different applications.
In most cases, the computer manufacturer has already configured the system to use the maximum amount of graphics memory that is possible for your particular computer model. Sometimes you can change these values through BIOS level depending on the options provided by the system manufacturer in the setup screen. You may want to check with your computer manufacturer if there are tips on increasing the amount of graphics memory for your computer model.
If you are interested in additional information see graphics memory frequently asked questions: