drm/i915: handle FIFO oversubsription correctly

If you're pushing a plane hard (i.e. you need most or all of the FIFO
entries just to cover your frame refresh latency), the watermark level
may end up being negative.  So fix up the signed vs. unsigned math in
the calculation function to handle this correctly, giving all available
FIFO entries to such a configuration.
Reported-by: Eric Anholt <eric@anholt.net>
Tested-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -1674,7 +1674,7 @@ static unsigned long intel_calculate_wm(unsigned long clock_in_khz,
 					int pixel_size,
 					unsigned long latency_ns)
 {
-	unsigned long entries_required, wm_size;
+	long entries_required, wm_size;
 
 	entries_required = (clock_in_khz * pixel_size * latency_ns) / 1000000;
 	entries_required /= wm->cacheline_size;
@@ -1685,9 +1685,10 @@ static unsigned long intel_calculate_wm(unsigned long clock_in_khz,
 
 	DRM_DEBUG("FIFO watermark level: %d\n", wm_size);
 
-	if (wm_size > wm->max_wm)
+	/* Don't promote wm_size to unsigned... */
+	if (wm_size > (long)wm->max_wm)
 		wm_size = wm->max_wm;
-	if (wm_size == 0)
+	if (wm_size <= 0)
 		wm_size = wm->default_wm;
 	return wm_size;
 }