drm/amdkfd: Fix doorbell initialization and finalization

Handle errors in doorbell aperture initialization instead of BUG_ON.
iounmap doorbell aperture during finalization.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>