acer-wmi: add quirk table for video backlight vendor mode

There have some acer laptop have broken _BCM implemenation, the AML
code wrote value to EC register but firmware didn't change brighenss.

Fortunately, the brightness control works on those machines with
vendor mode. So, add quirk table for video backlight vendor mode
and unregister acpi video interface on those machines.

Tested on Acer TravelMate 4750

Cc: Carlos Corbacho <[e-mail]>
Cc: Matthew Garrett <[e-mail]>
Cc: Dmitry Torokhov <[e-mail]>
Cc: Corentin Chary <[e-mail]>
Cc: Thomas Renninger <[e-mail]>
Signed-off-by: Lee, Chun-Yi <[e-mail]>
Signed-off-by: Matthew Garrett <[e-mail]>

drivers/platform/x86/Kconfig

···
 	depends on RFKILL || RFKILL = n
 	depends on ACPI_WMI
 	select INPUT_SPARSEKMAP
+	# Acer WMI depends on ACPI_VIDEO when ACPI is enabled
+	# but for select to work, need to select ACPI_VIDEO's dependencies, ick
+        select VIDEO_OUTPUT_CONTROL if ACPI
+        select ACPI_VIDEO if ACPI
 	---help---
 	  This is a driver for newer Acer (and Wistron) laptops. It adds
 	  wireless radio and bluetooth control, and on some laptops,

drivers/platform/x86/acer-wmi.c

···
 #include <linux/input/sparse-keymap.h>
 
 #include <acpi/acpi_drivers.h>
+#include <acpi/video.h>
 
 MODULE_AUTHOR("Carlos Corbacho");
 MODULE_DESCRIPTION("Acer Laptop WMI Extras Driver");
···
 			DMI_MATCH(DMI_PRODUCT_NAME, "0687A31"),
 		},
 		.driver_data = &quirk_fujitsu_amilo_li_1718,
+	},
+	{}
+};
+
+static int video_set_backlight_video_vendor(const struct dmi_system_id *d)
+{
+	interface->capability &= ~ACER_CAP_BRIGHTNESS;
+	pr_info("Brightness must be controlled by generic video driver\n");
+	return 0;
+}
+
+static const struct dmi_system_id video_vendor_dmi_table[] = {
+	{
+		.callback = video_set_backlight_video_vendor,
+		.ident = "Acer TravelMate 4750",
+		.matches = {
+			DMI_MATCH(DMI_BOARD_VENDOR, "Acer"),
+			DMI_MATCH(DMI_PRODUCT_NAME, "TravelMate 4750"),
+		},
 	},
 	{}
 };
···
 	set_quirks();
 
 	if (acpi_video_backlight_support()) {
-		interface->capability &= ~ACER_CAP_BRIGHTNESS;
-		pr_info("Brightness must be controlled by generic video driver\n");
+		if (dmi_check_system(video_vendor_dmi_table)) {
+			acpi_video_unregister();
+		} else {
+			interface->capability &= ~ACER_CAP_BRIGHTNESS;
+			pr_info("Brightness must be controlled by "
+				"acpi video driver\n");
+		}
 	}
 
 	if (wmi_has_guid(WMID_GUID3)) {