drm: cirrus: fix warnings

Regular nightly randconfig build testing discovered these warnings:

drivers/gpu/drm/cirrus/cirrus_drv.c:79:12: warning: 'cirrus_pm_suspend' defined but not used [-Wunused-function]
drivers/gpu/drm/cirrus/cirrus_drv.c:96:12: warning: 'cirrus_pm_resume' defined but not used [-Wunused-function]

Fix these by adding the same condition that SET_SYSTEM_SLEEP_PM_OPS()
uses.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>