drm/ast: Fixed the casting issue reported by sparse

V1:
1.Fixed sparse:cast truncates bits form constant value ()cast
  truncates bits from constant value (ffffffffffffff00 becomes 0)

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220623083116.35365-1-kuohsiang_chou@aspeedtech.com