drm/hisilicon: Use managed mode-config init

Using drmm_mode_config_init() sets up managed release of modesetting
resources.

v2:
Remove the unused structure member variable mode_config_initialized.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1607339778-20460-2-git-send-email-tiantao6@hisilicon.com