drm/amdkfd: fix a potential double free in pqm_create_queue

Set *q to NULL on errors, otherwise pqm_create_queue would free it
again.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>