drm/amdgpu: fix a compiling error in old kernels

This patch is used to fix following compiling error that occurs in
some old kernels.

 error: ‘for’ loop initial declarations are only allowed in C99 mode
    for (int i = 0; i < dc->res_pool->res_cap->num_dsc; i++) {
Signed-off-by: Asher Song <Asher.Song@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn32/dcn32_hwseq.c

@@ -1402,8 +1402,9 @@ void dcn32_update_dsc_pg(struct dc *dc,
 		bool safe_to_disable)
 {
 	struct dce_hwseq *hws = dc->hwseq;
+	int i;
 
-	for (int i = 0; i < dc->res_pool->res_cap->num_dsc; i++) {
+	for (i = 0; i < dc->res_pool->res_cap->num_dsc; i++) {
 		struct display_stream_compressor *dsc = dc->res_pool->dscs[i];
 		bool is_dsc_ungated = hws->funcs.dsc_pg_status(hws, dsc->inst);