drm/i915: Use AUX_CH_USBCn for the RKL VBT AUX CH setup

As with the VBT DVO port, RKL uses PHY based mapping for the
VBT AUX CH. Adjust the code to use the new AUX_USBCn names
and add a comment to explain the situation.
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201028213323.5423-7-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_bios.c

@@ -2664,12 +2664,16 @@ enum aux_ch intel_bios_port_aux_ch(struct drm_i915_private *dev_priv,
 		aux_ch = AUX_CH_B;
 		break;
 	case DP_AUX_C:
+		/*
+		 * RKL/DG1 VBT uses PHY based mapping. Combo PHYs A,B,C,D
+		 * map to DDI A,B,TC1,TC2 respectively.
+		 */
 		aux_ch = (IS_DG1(dev_priv) || IS_ROCKETLAKE(dev_priv)) ?
-			AUX_CH_D : AUX_CH_C;
+			AUX_CH_USBC1 : AUX_CH_C;
 		break;
 	case DP_AUX_D:
 		aux_ch = (IS_DG1(dev_priv) || IS_ROCKETLAKE(dev_priv)) ?
-			AUX_CH_E : AUX_CH_D;
+			AUX_CH_USBC2 : AUX_CH_D;
 		break;
 	case DP_AUX_E:
 		aux_ch = AUX_CH_E;