drm/i915: Enable GSE interrupt on BDW+

We've never actually enabled or unmasked the GSE interrupt on BDW+,
even though the interrupt handler was always prepared for it.
Let's enable it and see what happens.

Credit to Mark Kettenis who fixed this in the OpenBSD fork of the
driver. He reports that it fixed the "ACPI _BCM/_BCQ-based
brightness mechanism on a MacBookPro12,1 and a 3rd gen Lenovo X1
Carbon" for them.

Mark says:
"FWIW, this *is* needed if you want ACPI-based backlight control to
 work.  On Linux you probably don't notice, since "hardware" backlight
 control is preferred over "firmware" or "platform" backlight control.

 It would help me if this did land in the Linux tree though, as it will
 make future imports of the i915 driver into OpenBSD easier."

So even though we don't really need this, let's put it in to help Mark
with future porting efforts. Should be harmless to have it enabled in
any case.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
References: http://lists.freedesktop.org/archives/intel-gfx/2015-December/081799.htmlReported-by: Mark Kettenis <mark.kettenis@xs4all.nl>
Cc: Mark Kettenis <mark.kettenis@xs4all.nl>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463649283-28698-1-git-send-email-ville.syrjala@linux.intel.comAcked-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_irq.c

@@ -3794,6 +3794,7 @@ static void gen8_de_irq_postinstall(struct drm_i915_private *dev_priv)
 	uint32_t de_pipe_enables;
 	u32 de_port_masked = GEN8_AUX_CHANNEL_A;
 	u32 de_port_enables;
+	u32 de_misc_masked = GEN8_DE_MISC_GSE;
 	enum pipe pipe;
 
 	if (INTEL_INFO(dev_priv)->gen >= 9) {
@@ -3829,6 +3830,7 @@ static void gen8_de_irq_postinstall(struct drm_i915_private *dev_priv)
 					  de_pipe_enables);
 
 	GEN5_IRQ_INIT(GEN8_DE_PORT_, ~de_port_masked, de_port_enables);
+	GEN5_IRQ_INIT(GEN8_DE_MISC_, ~de_misc_masked, de_misc_masked);
 }
 
 static int gen8_irq_postinstall(struct drm_device *dev)