win32u: Complete support for display modes in nulldrv.
With this series it's now possible to run and pass user32:monitor and user32:sysparams tests with nulldrv, and so most user32 tests (except for a few desktop cursor position tests). This still requires some prefix configuration to enable the nulldrv driver, or a change like rbernon/wine@753368ad to enable it when DISPLAY environment variable is unset.
This then shows that some of the user32 tests are failing with winex11 but passing with nulldrv, as in rbernon/wine@6d5f4109.