amdgpu: Destroy fd_hash table when the last device is removed.

Fixes memory leak on module unload.
Analogous to mesa commit of the same name.
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>