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 4add75c4
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 12e8ba25
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>