drm/i915: quirk asserts controllable backlight presence, overriding VBT

commit c675949ec58ca50d5a3ae3c757892f1560f6e896
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Wed Apr 9 11:31:37 2014 +0300

    drm/i915: do not setup backlight if not available according to VBT

caused a regression on machines with a misconfigured VBT. Add a quirk to
assert the presence of a controllable backlight. Use it to ignore the VBT
backlight presence check during backlight setup.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79813
Tested-by: James Duley <jagduley@gmail.com>
Tested-by: Michael Mullin <masmullin@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Scot Doyle <lkml14@scotdoyle.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Cc: stable@vger.kernel.org # 3.15 only
[danvet: Add cc: stable because the regressing commit is in 3.15.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index a47fbf6..374f964 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -656,6 +656,7 @@
 #define QUIRK_PIPEA_FORCE (1<<0)
 #define QUIRK_LVDS_SSC_DISABLE (1<<1)
 #define QUIRK_INVERT_BRIGHTNESS (1<<2)
+#define QUIRK_BACKLIGHT_PRESENT (1<<3)
 
 struct intel_fbdev;
 struct intel_fbc_work;