disp: msm: sde: fix null check for encoder dsc disable

There are some scenarios where connectors can be
null during encoder dsc disable. Update the null
check to avoid error message during these scenarios.

Change-Id: I598a644f19aecf0b7d8c989a75575329903678dd
Signed-off-by: Krishna Manikandan <mkrishn@codeaurora.org>