drm/i915: Check if BIOS enabled dual-channel LVDS on 8xx, not only on 9xx

Commit 0c2e39525b3b53a97a0202c5f35058147e53977e is not sufficient to
get fd.o bug #20115 fixed.
In addition intel_find_best_PLL() must not only rely on BIOS settings
for i9xx chips but also for i8xx, so drop the IS_I9XX() check.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Eric Anholt <eric@anholt.net>