drm/amd/display: handle "18" case in TruncToValidBPP

Handle 18 DecimalBPP like other cases
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c

@@ -3380,6 +3380,8 @@ static unsigned int TruncToValidBPP(
 				return 30;
 			else if (DecimalBPP >= 24 && (DesiredBPP == 0 || DesiredBPP == 24))
 				return 24;
+			else if (DecimalBPP >= 18 && (DesiredBPP == 0 || DesiredBPP == 18))
+				return 18;
 			else
 				return BPP_INVALID;
 		}