drm/ast: Do not include <linux/export.h>

Fix the compile-time warning

  drivers/gpu/drm/ast/ast_mode.c: warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250612084257.200907-1-tzimmermann@suse.de