mdss: display: turn off intr_callback with correct intf_num

Same intf_num is needed to register and un-register call back
function. Make sure same intf_num is used at both register callback
at cmd_start() and un-register at cmd_stop().

Change-Id: Id6de434d04a94b2467047962d04ca69e60477dc2
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>