drm: correct function name drm_legacy_ctxbitmap_flush()

Fix the following make W=1 kernel build warning:

  drivers/gpu/drm/drm_context.c:136: warning: expecting prototype for drm_ctxbitmap_flush(). Prototype was for drm_legacy_ctxbitmap_flush() instead

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210513071918.1728535-1-yangyingliang@huawei.com