drm/hisilicon: Use managed VRAM-helper initialization

updated to use drmm_vram_helper_init()

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1606964953-24309-1-git-send-email-tiantao6@hisilicon.com