drm/hisilicon: Using the to_hibmc_drm_private to convert

Using the to_hibmc_drm_private to convert over all uses of
dev_private, and fix a little formatting issue.

v2:
fixed the commit message.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1601282978-45534-1-git-send-email-tiantao6@hisilicon.com