drm/hisilicon: Code refactoring for hibmc_drv_vdac

code refactoring for hibmc_drv_vdac.c, no actual function changes.

v2:
remove the debug message.

v3:
embedding connector and encoder in struct hibmc_drm_private.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1593680081-60313-1-git-send-email-tiantao6@hisilicon.com