drm/amdgpu: add a missing lock for AMDGPU_SCHED

mgr->ctx_handles should be protected by mgr->lock.

v2: improve commit message
v3: add a Fixes tag

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Fixes: 52c6a62c

 ("drm/amdgpu: add interface for editing a foreign process's priority v3")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>