drm: optimize drm_framebuffer_remove

Now that all framebuffer usage is properly refcounted, we are no
longer required to hold the modeset locks while dropping the last
reference. Hence implemented a fastpath which avoids the potential
stalls associated with grabbing mode_config.lock for the case where
there's no other reference around.

Explain in a big comment why it is safe. Also update kerneldocs with
the new locking rules around drm_framebuffer_remove.

Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>