ACPI / video: Load the module even if ACPI is disabled

i915.ko depends upon the acpi/video.ko module and so refuses to load if
ACPI is disabled at runtime if for example the BIOS is broken beyond
repair. acpi/video provides an optional service for i915.ko and so we
should just allow the modules to load, but do no nothing in order to let
the machines boot correctly.

Reported-by: Bill Augur <bill-auger@programmer.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: All applicable <stable@vger.kernel.org>
Acked-by: Aaron Lu <aaron.lu@intel.com>
[ rjw: Fixed up the new comment in acpi_video_init() ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video.c

@@ -2176,6 +2176,17 @@ EXPORT_SYMBOL(acpi_video_unregister_backlight);
 
 static int __init acpi_video_init(void)
 {
+	/*
+	 * Let the module load even if ACPI is disabled (e.g. due to
+	 * a broken BIOS) so that i915.ko can still be loaded on such
+	 * old systems without an AcpiOpRegion.
+	 *
+	 * acpi_video_register() will report -ENODEV later as well due
+	 * to acpi_disabled when i915.ko tries to register itself afterwards.
+	 */
+	if (acpi_disabled)
+		return 0;
+
 	dmi_check_system(video_dmi_table);
 
 	if (intel_opregion_present())