drm/amdkfd: Add NULL-pointer check

top_dev->gpu is NULL for CPUs. Avoid dereferencing it if NULL.

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>