drm/atmel-hlcdc: Compile suspend/resume for PM_SLEEP only

If PM is enabled but PM_SLEEP is disabled, the suspend/resume functions
are still unused and produce a compiler warning.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: <stable@vger.kernel.org> # 4.1+