Documentation: drm: Remove dangling pointer from drm-mm.rst

Fixes htmldocs build error:
Error: Cannot open file ../drivers/gpu/drm/drm_global.c

Fixes: 2bb42410 ("drm: Remove drm_global.{c,h} v2")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Christian König <christian.koenig@amd.com>
Cc: Junwei Zhang <Jerry.Zhang@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129155522.33749-1-sean@poorly.run