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

This reverts commit 0f3ee801

.

Enabling LVDS on pipe A was causing excessive wakeups on otherwise idle
systems due to i915 interrupts. So restrict the LVDS to pipe B once more,
whilst the issue is properly diagnosed.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=16307
Reported-and-tested-by: Enrico Bandiello <enban@postal.uv.es>
Poked-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Adam Jackson <ajax@redhat.com>
Cc: stable@kernel.org