drm/i915/dp: return errors from rate_to_index()

We shouldn't silently use the first element if we can't find the rate
we're looking for. Make rate_to_index() more generally useful, and
fallback to the first element in the caller, with a big warning.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/8a6e83b7bf35da0cbbc703ae157944107ff145be.1490712890.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -1544,9 +1544,9 @@ static int rate_to_index(const int *rates, int len, int rate)
 
 	for (i = 0; i < len; i++)
 		if (rate == rates[i])
-			break;
-
 			return i;
+
+	return -1;
 }
 
 int
@@ -1564,8 +1564,13 @@ intel_dp_max_link_rate(struct intel_dp *intel_dp)
 
 int intel_dp_rate_select(struct intel_dp *intel_dp, int rate)
 {
-	return rate_to_index(intel_dp->sink_rates, intel_dp->num_sink_rates,
+	int i = rate_to_index(intel_dp->sink_rates, intel_dp->num_sink_rates,
 			      rate);
+
+	if (WARN_ON(i < 0))
+		i = 0;
+
+	return i;
 }
 
 void intel_dp_compute_rate(struct intel_dp *intel_dp, int port_clock,