drm/i915/dp: use known correct array size in rate_to_index

I can't think of a real world bug this could cause now, but this will be
required in follow-up work. While at it, change the parameter order to
be slightly more sensible.

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/ff5b08f45a72c2247f5326b080027e2f5d8cc4ee.1490712890.git.jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -1538,12 +1538,12 @@ bool intel_dp_read_desc(struct intel_dp *intel_dp)
 	return true;
 }
 
-static int rate_to_index(int find, const int *rates)
+static int rate_to_index(const int *rates, int len, int rate)
 {
-	int i = 0;
+	int i;
 
-	for (i = 0; i < DP_MAX_SUPPORTED_RATES; ++i)
-		if (find == rates[i])
+	for (i = 0; i < len; i++)
+		if (rate == rates[i])
 			break;
 
 	return i;
@@ -1564,7 +1564,8 @@ 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(rate, intel_dp->sink_rates);
+	return rate_to_index(intel_dp->sink_rates, intel_dp->num_sink_rates,
+			     rate);
 }
 
 void intel_dp_compute_rate(struct intel_dp *intel_dp, int port_clock,