drm: Check locking in drm_for_each_fb

Ever since framebuffers are reference counted we have a special lock
for the global fb list. Make sure users of that list do hold that
lock when using the new iterators.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>