drm/i915: make system freeze support depend on CONFIG_ACPI_SLEEP

To achieve further power savings during system freeze (aka connected
standby, or s0ix) we have to send a PCI_D1 opregion notification. As
the information about the state we're entering (system freeze,
suspend to ram or suspend to disk) is only available through the ACPI
subsystem, make this support depend on the relevant kconfig option.
Things will still work if this option isn't set, albeit with less than
optimial power saving.

This also fixes a compile breakage when the option is not set introduced
in

commit e5747e3a
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Jun 12 08:35:47 2014 -0700

    drm/i915: send proper opregion notifications on suspend/resume

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>