accel/qaic: Grab ch_lock during QAIC_ATTACH_SLICE_BO

During QAIC_ATTACH_SLICE_BO, we associate a BO to its DBC. We need to
grab the dbc->ch_lock to make sure that DBC does not goes away while
QAIC_ATTACH_SLICE_BO is still running.

Fixes: ff13be83 ("accel/qaic: Add datapath")
Signed-off-by: Pranjal Ramajor Asha Kanojiya <quic_pkanojiy@quicinc.com>
Reviewed-by: Carl Vanderlip <quic_carlv@quicinc.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230517193540.14323-5-quic_jhugo@quicinc.com