drm/i915: Add invert-brightness quirk for Thundersoft TST178 tablet

The Thundersoft TST178 tablet uses a DSI panel with an external PWM
controller (as all DSI panels do). But unlike other DSI panels a duty-cycle
of 100% turns the backlight off and 0% sets it to maximum brightness.

I've checked the VBT and there is a BDB_LVDS_BACKLIGHT section, but
it does not set the active_low_pwm flag. This tablet re-uses the main
PCI vendor and product ids for the subsystem ids, so I see no other option
then to add a DMI based quirk to fix this.

Note that the PWM backlight code in intel_panel.c currently does not honor
the vbt.active_low_pwm flag, but that does not matter in this case.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200221172927.510027-2-hdegoede@redhat.com

drivers/gpu/drm/i915/display/intel_quirks.c

@@ -82,6 +82,16 @@ static const struct intel_dmi_quirk intel_dmi_quirks[] = {
 					    DMI_MATCH(DMI_PRODUCT_NAME, ""),
 				},
 			},
+			{
+				.callback = intel_dmi_reverse_brightness,
+				.ident = "Thundersoft TST178 tablet",
+				/* DMI strings are too generic, also match on BIOS date */
+				.matches = {DMI_EXACT_MATCH(DMI_BOARD_VENDOR, "AMI Corporation"),
+					    DMI_EXACT_MATCH(DMI_BOARD_NAME, "Aptio CRB"),
+					    DMI_EXACT_MATCH(DMI_PRODUCT_NAME, "To be filled by O.E.M."),
+					    DMI_EXACT_MATCH(DMI_BIOS_DATE, "04/15/2014"),
+				},
+			},
 			{ }  /* terminating entry */
 		},
 		.hook = quirk_invert_brightness,