drm/qxl: remove unused declaration

Some functions are never used by the driver,
removing the functions declaration, it can be reducing program size,
and improving code readability and maintainability.

Signed-off-by: heminhong <heminhong@kylinos.cn>
Link: https://lore.kernel.org/r/20231110055031.57360-1-heminhong@kylinos.cn


Signed-off-by: Maxime Ripard <mripard@kernel.org>