drm/amdkfd: Fix sdma queue allocate race condition

SDMA queue allocation requires the dqm lock as it modify
the global dqm members. Enclose it in the dqm_lock.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Philip Yang <philip.yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>