DRM: Armada: implement lastclose() for fbhelper

Call drm_fb_helper_restore_fbdev_mode() upon last close so that in the
event of the X server crashing, we have some kind of mode restored.

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>