drm/amdkfd: fix a potential NULL pointer dereference (v2)

alloc_workqueue is not checked for errors and as a result,
a potential NULL dereference could occur.

v2 (Felix Kuehling):
* Fix compile error (kfifo_free instead of fifo_free)
* Return proper error code

Signed-off-by: Allen Pais <allen.pais@oracle.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
