msm: mdss: Fix null pointer reference during debug init

Some targets doesn't seem to have vbif_nrt_phys in dtsi
file but we still register the debug node for vbif_nrt_reg
and off without checking whether it exists or not. Ensure
this is checked to avoid null pointer reference.

Change-Id: Icce8823f80e586bc4827d8db18dceb2fb866c2f9
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>