linuxkpi: Update `vga_client_register()` and add `vga_client_unregister()`

[dumbbell]

Summary

For vga_client_register(), the API is modified twice in a row. To keep the API compatible with all commits in the DRM driver, we introduce two LINUXKPI_VERSION version bumps.

This is part of the update of the DRM drivers to Linux 5.15.