acpithinkpad: re-assert backlight control

Revision 1.67 made acpithinkpad not take over screen backlight
control and let inteldrm or some other driver handle it.  This was
done to support fine-grained levels of backlight control rather than
the dozen steps that the native EC interface supports.

Unfortunately this has the drawback of the EC getting confused about
what the backlight level actually is and it still tries to change
the backlight automatically in response to Fn+F# keys or plug/unplug
events.  Notably if you have the laptop plugged in with a backlight
value of something high, then unplug it and later lower it with
xbacklight, when you plug it back in, the EC automatically adjusts
the backlight up to that high value.

I don't know how to tell the EC to stop adjusting the backlight
automatically on power changes, so I would like to revert this
change and go back to only having native EC-controlled backlight
with xbacklight or wsconsctl.

patrick@ confirmed that he can still change backlight values on the
ThinkPad X395 with this backed out, even though part of the reason
for this initial change was to help that machine.

Author: jcs

sys/dev/acpi/acpithinkpad.c

@@ -364,13 +364,15 @@ thinkpad_attach(struct device *parent, struct device *self, void *aux)
 		wskbd_set_backlight = thinkpad_set_kbd_backlight;
 	}
 
-	/* On version 2 and newer, let *drm or acpivout control brightness */
-	if (sc->sc_hkey_version == THINKPAD_HKEY_VERSION1 &&
-	    (aml_evalinteger(sc->sc_acpi, sc->sc_devnode, "PBLG",
-	    0, NULL, &sc->sc_brightness) == 0)) {
+	if (
+#if 0
+	sc->sc_hkey_version == THINKPAD_HKEY_VERSION1 &&
+#endif
+	    thinkpad_get_brightness(sc) == 0) {
 		ws_get_param = thinkpad_get_param;
 		ws_set_param = thinkpad_set_param;
 	}
+	/* On version 2 and newer, let *drm or acpivout control brightness */
 
 	memset(&arg, 0, sizeof(arg));
 	arg.type = AML_OBJTYPE_INTEGER;