drm/amd/display: Separate set_gsl from set_gsl_source_select

[Why/How]
Separate the checks for set_gsl and set_gsl_source_select, since
source_select may not be implemented/necessary.

Reviewed-by: Nevenko Stupar <[e-mail]>
Signed-off-by: Ilya Bakoulin <[e-mail]>
Signed-off-by: Ray Wu <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c

···
 	}
 
 	/* at this point we want to program whether it's to enable or disable */
-	if (pipe_ctx->stream_res.tg->funcs->set_gsl != NULL &&
-		pipe_ctx->stream_res.tg->funcs->set_gsl_source_select != NULL) {
+	if (pipe_ctx->stream_res.tg->funcs->set_gsl != NULL) {
 		pipe_ctx->stream_res.tg->funcs->set_gsl(
 			pipe_ctx->stream_res.tg,
 			&gsl);
-
-		pipe_ctx->stream_res.tg->funcs->set_gsl_source_select(
-			pipe_ctx->stream_res.tg, group_idx,	enable ? 4 : 0);
+		if (pipe_ctx->stream_res.tg->funcs->set_gsl_source_select != NULL)
+			pipe_ctx->stream_res.tg->funcs->set_gsl_source_select(
+				pipe_ctx->stream_res.tg, group_idx, enable ? 4 : 0);
 	} else
 		BREAK_TO_DEBUGGER();
 }