drm/mode: reduce scope of fb_lock in framebuffer init

We don't need to hold the fb lock around the initialisation,
only around the list manipulaton.

So do the lock hold only around the register for now.

From Daniel:
Previously fb refcounting, and especially the weak reference
(kref_get_unless_zero) used in fb lookups have been protected by fb_lock.
But with the refactoring to share refcounting in the drm_mode_object base
class that switched to being protected by idr_mutex, which means fb_lock
critical sections can be reduced.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>