drm/i915: Fix divide by zero on watermark update

Fix divide by zero if we end up updating the watermarks
with zero dotclock.

This is a stop gap measure to allow module load in cases
where our state keeping fails.

v2: WARN_ON added (Paulo)

Cc: Paulo Zanoni <przanoni@gmail.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -3316,8 +3316,10 @@ skl_compute_linetime_wm(struct drm_crtc *crtc, struct skl_pipe_wm_parameters *p)
 	if (!to_intel_crtc(crtc)->active)
 		return 0;
 
-	return DIV_ROUND_UP(8 * p->pipe_htotal * 1000, p->pixel_rate);
+	if (WARN_ON(p->pixel_rate == 0))
+		return 0;
 
+	return DIV_ROUND_UP(8 * p->pipe_htotal * 1000, p->pixel_rate);
 }
 
 static void skl_compute_transition_wm(struct drm_crtc *crtc,