drm/hisilicon: Code refactoring for hibmc_drm_drv

Use the devm_drm_dev_alloc provided by the drm framework to alloc
a structure hibmc_drm_private.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1606901212-8214-2-git-send-email-tiantao6@hisilicon.com