DRM: Armada: destroy framebuffer after helper

Destroy the framebuffer only after the helper, since the helper may
still be referencing the framebufer at this point.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>