drm/atmel-hlcdc: switch to drm_*{get,put} helpers

drm_*_reference() and drm_*_unreference() functions are just
compatibility alias for drm_*_get() and drm_*_put() and should not be
used by new code. So convert all users of compatibility functions to use
the new APIs.

Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1501761585-11757-6-git-send-email-cakturk@gmail.com