drm/i915: fix inconsistent brightness after resume

commit 6dda730e
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Tue Jun 24 18:27:40 2014 +0300

    drm/i915: respect the VBT minimum backlight brightness

introduced a bug which resulted in inconsistent brightness levels on
different machines. If a suspended was entered with the screen off some
machines would resume with the screen at minimum brightness and others
at maximum brightness.

The following commands can be used to produce this behavior.

  xset dpms force off
  sleep 1
  sudo systemctl suspend
  (resume ...)

The root cause of this problem is a comparison which checks to see if
the backlight level is zero when the panel is enabled.  If it is zero,
it is set to the maximum level.  Unfortunately, not all machines have a
minimum level of zero. On those machines the level is left at the
minimum instead of begin set to the maximum.

Fix the bug by updating the comparison to check for the minimum
backlight level instead of zero.  Also, expand the comparison for
the possible case when the level is less than the minimum.

Fixes: 6dda730e ("respect the VBT minimum backlight brightness")
Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -962,7 +962,7 @@ void intel_panel_enable_backlight(struct intel_connector *connector)
 
 	WARN_ON(panel->backlight.max == 0);
 
-	if (panel->backlight.level == 0) {
+	if (panel->backlight.level <= panel->backlight.min) {
 		panel->backlight.level = panel->backlight.max;
 		if (panel->backlight.device)
 			panel->backlight.device->props.brightness =