drm/atmel-hlcdc: Fix suspend/resume implementation

The current suspend resume implementation is assuming register values are
kept when entering suspend, which is no longer the case with the
suspend-to-RAM on the sama5d2.

While at it, switch to the generic infrastructure to enter suspend mode
(drm_atomic_helper_suspend/resume()).

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1488371461-22243-1-git-send-email-boris.brezillon@free-electrons.com