drm/amd/display: Add writeback_config to VBA vars

Adding writeback_config enum to vba_vars_st, replacing old flag.
Initialize to dm_normal.

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml/display_mode_enums.h

@@ -147,4 +147,10 @@ enum dm_validation_status {
 	DML_FAIL_V_RATIO_PREFETCH,
 };
 
+enum writeback_config {
+	dm_normal,
+	dm_whole_buffer_for_single_stream_no_interleave,
+	dm_whole_buffer_for_single_stream_interleave,
+};
+
 #endif