drm/i915: fix build errors when ACPI is not enabled

Fix build errors when ACPI is not enabled by adding function stubs:

../drivers/gpu/drm/i915/i915_drv.c: In function 'i915_drm_suspend':
../drivers/gpu/drm/i915/i915_drv.c:635:2: error: implicit declaration of function 'intel_opregion_unregister' [-Werror=implicit-function-declaration]
  intel_opregion_unregister(dev_priv);
../drivers/gpu/drm/i915/i915_drv.c: In function 'i915_drm_resume':
../drivers/gpu/drm/i915/i915_drv.c:798:2: error: implicit declaration of function 'intel_opregion_register' [-Werror=implicit-function-declaration]
  intel_opregion_register(dev_priv);

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Fixes: 03d92e47 ("drm/i915/opregion: Rename init/fini functions to register/unregister")
Cc: drm-intel-fixes@lists.freedesktop.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[Jani: dropped the stale init/fini declarations]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467028399-9965-1-git-send-email-jani.nikula@intel.com