drm: atmel-hlcdc: Add PM suspend/resume support

On suspend: switch off CRTC if not already suspended with runtime PM

On resume: switch on CRTC if we were not already suspended from runtime
PM while suspending.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>