drm/nvc0/fb: fix crash when different mutex is used to protect same list

Fixes regression introduced in commit 861d2107


"drm/nouveau/fb: merge fb/vram and port to subdev interfaces"

nv50_fb_vram_{new,del} functions were changed to use
nouveau_subdev->mutex instead of the old nouveau_mm->mutex.
nvc0_fb_vram_new still uses the nouveau_mm->mutex, but nvc0 doesn't
have its own fb_vram_del function, using nv50_fb_vram_del instead.
Because of this, on nvc0 a different mutex ends up being used to protect
additions and deletions to the same list.

This patch is a -stable candidate for 3.7.

Signed-off-by: Aleksi Torhamo <aleksi@torhamo.net>
Reported-by: Roy Spliet <r.spliet@student.tudelft.nl>
Tested-by: Roy Spliet <r.spliet@student.tudelft.nl>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org