drm/amdkfd: Dereference null return value

In the function pqm_uninit there is a call-assignment of "pdd =
kfd_get_process_device_data" which could be null, and this value was
later dereferenced without checking.

Fixes: fb910658 ("drm/amdkfd: Refactor queue wptr_bo GART mapping")
Signed-off-by: Andrew Martin <Andrew.Martin@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org