drm/hisilicon/hibmc: Export VRAM MM information to debugfs

This change makes information about VRAM consumption available on
debugfs. See

  /sys/kernel/debug/dri/0/vram-mm

for an overview of how VRAM is being used.

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/20191203083819.6643-6-tzimmermann@suse.de