drm/i915: disable_shared_pll doesn't work on pre-gen5

Looks like

commit eddfcbcd
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Mon Jun 15 12:33:53 2015 +0200
    drm/i915: Update less state during modeset.

introduced the unconditional calling of disable_shared_dpll, but didn't
fix up pre-gen5 to avoid the BUG_ON at the top of the function.

So change the BUG_ON into a gen check (alternately we could move the
BUG_ON until later, since we shouldn't have a pll struct here either,
but this seems clearer to read).

This fixes a crash on load on my x200s platform.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -1936,7 +1936,9 @@ static void intel_disable_shared_dpll(struct intel_crtc *crtc)
 	struct intel_shared_dpll *pll = intel_crtc_to_shared_dpll(crtc);
 
 	/* PCH only available on ILK+ */
-	BUG_ON(INTEL_INFO(dev)->gen < 5);
+	if (INTEL_INFO(dev)->gen < 5)
+		return;
+
 	if (pll == NULL)
 		return;