drm/hisilicon/hibmc: Replace struct hibmc_framebuffer with generic code

The hibmc driver's struct hibmc_framebuffer stores a DRM framebuffer
with an associated GEM object. This functionality is also provided by
generic code. Switch hibmc over.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203083819.6643-3-tzimmermann@suse.de