Refactor video attributes

[bkoropoff]

gbaHD no longer needs custom video timings, but this refactoring makes it easier if anyone ever does.

• Abstract video attributes and video ID code lookup into a package
• All video attributes are now static values rather than signals within the hdmi block. The width/height output signals are removed since they are available by accessing hdmi localparams or through the hdmi_attr package.
• Allow using custom video attributes if desired