amdgpu: use NULL instead of 0 for pointer

In the AMD powerplay driver, a pointer is checked for validity by
comparing against an integer '0', which causes a harmless warning
when building with "make W=1":

drivers/gpu/drm/amd/amdgpu/../powerplay/hwmgr/processpptables.c:1502:16: error: ordered comparison of pointer with integer zero [-Werror=extra]

This changes the code to the more conventional "if (pointer)" check.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/powerplay/hwmgr/processpptables.c

@@ -1502,7 +1502,7 @@ int get_number_of_vce_state_table_entries(
 	const ATOM_PPLIB_VCE_State_Table *vce_table =
 				    get_vce_state_table(hwmgr, table);
 
-	if (vce_table > 0)
+	if (vce_table)
 		return vce_table->numEntries;
 
 	return 0;