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.html
Reported-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.com
Acked-by: Jani Nikula <jani.nikula@intel.com>