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>