drm/ast: make ast_modeset static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

drivers/gpu/drm/ast/ast_drv.c:42:5: warning: symbol 'ast_modeset'
was not declared. Should it be static?

Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220926023253.739699-1-ruanjinjie@huawei.com