drm/i915: allow pipe A for lvds on gen4

Given the havoc the missing backlight pipe select code might have
caused, let's try to re-enabled pipe A support for lvds on gen4 hw.
Just to see what all blows up ...

Note though that

commit 4add75c43f39573edc884d46b7c2b7414f01171a
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sat Dec 4 17:49:46 2010 +0000

    drm/i915: Allow LVDS to be on pipe A for Ironlake+

claims that this caused tons of spurious wakeups somehow.

More details can be found in the old revert:

commit 12e8ba25ef52f19e7a42e61aecb3c1fef83b2a82
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Sep 7 23:39:28 2010 +0100

    Revert "drm/i915: Allow LVDS on pipe A on gen4+"

    Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=16307

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lvds.c

@@ -968,6 +968,8 @@ bool intel_lvds_init(struct drm_device *dev)
 	intel_encoder->clone_mask = (1 << INTEL_LVDS_CLONE_BIT);
 	if (HAS_PCH_SPLIT(dev))
 		intel_encoder->crtc_mask = (1 << 0) | (1 << 1) | (1 << 2);
+	else if (IS_GEN4(dev))
+		intel_encoder->crtc_mask = (1 << 0) | (1 << 1);
 	else
 		intel_encoder->crtc_mask = (1 << 1);