drm/hisilicon/hibmc: Switch to generic fbdev emulation

There's nothing special about hibmc's fbdev emulation that is not
provided by the generic implementation. Switch over and remove the
driver's code.

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/20191203083819.6643-2-tzimmermann@suse.de