ACPI / video: Only default only_lcd to true on Win8-ready _desktops_

Commit 5928c281 (ACPI / video: Default lcd_only to true on Win8-ready
and newer machines) made only_lcd default to true on all machines where
acpi_osi_is_win8() returns true, including laptops.

The purpose of this is to avoid the bogus / non-working acpi backlight
interface which many newer BIOS-es define on desktop machines.

But this is causing a regression on some laptops, specifically on the
Dell XPS 13 2013 model, which does not have the LCD flag set for its
fully functional ACPI backlight interface.

Rather then DMI quirking our way out of this, this commits changes the
logic for setting only_lcd to true, to only do this on machines with
a desktop (or server) dmi chassis-type.

Note that we cannot simply only check the chassis-type and not register
the backlight interface based on that as there are some laptops and
tablets which have their chassis-type set to "3" aka desktop. Hopefully
the combination of checking the LCD flag, but only on devices with
a desktop(ish) chassis-type will avoid the needs for DMI quirks for this,
or at least limit the amount of DMI quirks which we need to a minimum.

Fixes: 5928c281 (ACPI / video: Default lcd_only to true on Win8-ready and newer machines)
Reported-and-tested-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Cc: 4.15+ <stable@vger.kernel.org> # 4.15+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_video.c

@@ -2123,6 +2123,25 @@ static int __init intel_opregion_present(void)
 	return opregion;
 }
 
+static bool dmi_is_desktop(void)
+{
+	const char *chassis_type;
+
+	chassis_type = dmi_get_system_info(DMI_CHASSIS_TYPE);
+	if (!chassis_type)
+		return false;
+
+	if (!strcmp(chassis_type, "3") || /*  3: Desktop */
+	    !strcmp(chassis_type, "4") || /*  4: Low Profile Desktop */
+	    !strcmp(chassis_type, "5") || /*  5: Pizza Box */
+	    !strcmp(chassis_type, "6") || /*  6: Mini Tower */
+	    !strcmp(chassis_type, "7") || /*  7: Tower */
+	    !strcmp(chassis_type, "11"))  /* 11: Main Server Chassis */
+		return true;
+
+	return false;
+}
+
 int acpi_video_register(void)
 {
 	int ret = 0;
@@ -2143,8 +2162,12 @@ int acpi_video_register(void)
 	 * win8 ready (where we also prefer the native backlight driver, so
 	 * normally the acpi_video code should not register there anyways).
 	 */
-	if (only_lcd == -1)
-		only_lcd = acpi_osi_is_win8();
+	if (only_lcd == -1) {
+		if (dmi_is_desktop() && acpi_osi_is_win8())
+			only_lcd = true;
+		else
+			only_lcd = false;
+	}
 
 	dmi_check_system(video_dmi_table);