linuxkpi: Update vga_client_register() and add vga_client_unregister()

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.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38533

(cherry picked from commit 1c6d8146fdec8267712615c261bf6c8800f8ecfe)