drm/i915: fix backlight invert for non-zero minimum brightness

When we started following the backlight minimum brightness in
6dda730 ("drm/i915: respect the VBT minimum backlight brightness")
we overlooked the brightness invert quirk. Even if we invert the
brightness, we need to take the min limit into account. We probably
missed this because the invert has only been required on gen4 for proper
operation.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101127
Fixes: 6dda730 ("drm/i915: respect the VBT minimum backlight brightness")
Cc: Daniel Vetter <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Signed-off-by: Jani Nikula <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit e9d7486)
Signed-off-by: Jani Nikula <[email protected]>

drivers/gpu/drm/i915/intel_panel.c

@@ -469,7 +469,7 @@ static u32 intel_panel_compute_brightness(struct intel_connector *connector,
 
 	if (i915.invert_brightness > 0 ||
 	    dev_priv->quirks & QUIRK_INVERT_BRIGHTNESS) {
-		return panel->backlight.max - val;
+		return panel->backlight.max - val + panel->backlight.min;
 	}
 
 	return val;