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

Commit 0c2e3952 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: NBruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -666,7 +666,7 @@ intel_find_best_PLL(const intel_limit_t *limit, struct drm_crtc *crtc,
 	intel_clock_t clock;
 	int err = target;
 
-	if (IS_I9XX(dev) && intel_pipe_has_type(crtc, INTEL_OUTPUT_LVDS) &&
+	if (intel_pipe_has_type(crtc, INTEL_OUTPUT_LVDS) &&
 	    (I915_READ(LVDS)) != 0) {
 		/*
 		 * For LVDS, if the panel is on, just rely on its current