drm/armada: Use drm_fb_helper_lastclose() and _poll_changed()

This driver can use drm_fb_helper_lastclose() as its .lastclose callback.
It can also use drm_fb_helper_output_poll_changed() as its
.output_poll_changed callback.

Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171205182504.41923-4-noralf@tronnes.org