drm/amd/display: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./drivers/gpu/drm/amd/display/dc/clk_mgr/dcn31/dcn31_clk_mgr.c:643:35-36:
WARNING comparing pointer to 0.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: chongjiapeng <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/clk_mgr/dcn31/dcn31_clk_mgr.c

@@ -640,7 +640,7 @@ void dcn31_clk_mgr_construct(
 				sizeof(struct dcn31_watermarks),
 				&clk_mgr->smu_wm_set.mc_address.quad_part);
 
-	if (clk_mgr->smu_wm_set.wm_set == 0) {
+	if (!clk_mgr->smu_wm_set.wm_set) {
 		clk_mgr->smu_wm_set.wm_set = &dummy_wms;
 		clk_mgr->smu_wm_set.mc_address.quad_part = 0;
 	}