drm/amdkfd: Remove unused variable

kfd_topology_device->cache_count is not used by
other fucntions, so remove it.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>