drm/amd/display: Fix unsigned comparison to zero

Fixes coccicheck warning:

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c:1398:60-61:
WARNING: Unsigned expression compared with zero: j >= 0

Fixes: 23838777 ("drm/amd/display: fix rn soc bb update")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c

@@ -1384,7 +1384,8 @@ static void update_bw_bounding_box(struct dc *dc, struct clk_bw_params *bw_param
 	struct dcn21_resource_pool *pool = TO_DCN21_RES_POOL(dc->res_pool);
 	struct clk_limit_table *clk_table = &bw_params->clk_table;
 	struct _vcs_dpi_voltage_scaling_st clock_limits[DC__VOLTAGE_STATES];
-	unsigned int i, j, closest_clk_lvl;
+	unsigned int i, closest_clk_lvl;
+	int j;
 
 	// Default clock levels are used for diags, which may lead to overclocking.
 	if (!IS_DIAG_DC(dc->ctx->dce_environment)) {