amdgpu: Take a lock before removing devices from fd_tab hash table.

Close the file descriptors under lock as well.
v2: close fds after removing from hash table

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>