msm: mdss: fix vbif debug bus dump in log

Invalid memory access during vbif debug bus
dump in log leads to crash. This adds null
check before accessing debug bus memory.

Change-Id: I54b74ce1004c4246398f18cd1a262e29a0a20d20
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>