drm/nouveau: Drop dev->struct_mutex from fbdev init

Doesn't protect anything at all.

With this patch nouveau is completely dev->struct_mutex free!

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-6-git-send-email-daniel.vetter@ffwll.ch