disp: msm: sde: add checks to avoid null pointer dereference

This change adds required checks to avoid accessing null pointer
where debug bus is not initialised and dumping the debug bus points.

Change-Id: Ia46a4b95706f05e37be660f006c04b1e2e9f6848
Signed-off-by: Andhavarapu Karthik <kartkart@codeaurora.org>