drm/vram: add vram-mm debugfs file

Wire up drm_mm_print() for vram helpers, using a new
debugfs file, so one can see how vram is used:

   # cat /sys/kernel/debug/dri/0/vram-mm
   0x0000000000000000-0x0000000000000300: 768: used
   0x0000000000000300-0x0000000000000600: 768: used
   0x0000000000000600-0x0000000000000900: 768: used
   0x0000000000000900-0x0000000000000c00: 768: used
   0x0000000000000c00-0x0000000000004000: 13312: free
   total: 16384, used 3072 free 13312

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190904054740.20817-5-kraxel@redhat.com